linuxkit: Metadata panics when trying to read cdrom

Description

Metadata panics and quits when trying to read cloud-config from ISO cdrom with error:
panic: runtime error: index out of range [0] with length 0

Steps to reproduce the issue:

• Run linuxkit as a guest on libvirt qemu.
• Create iso with cloud-config on a host machine:
  mkisofs -output <isoDestination> -volid cidata -joliet -rock <userDataFileName> <metaDataFileName> <networkConfigFileName>
• Attach iso as a cdrom
• Run metadata -v

Describe the results you received:

metadata -v
DEBU[0000] cdrom devices to be checked: [/dev/sr0]      
DEBU[0000] block devices found: [/sys/class/block/loop0 /sys/class/block/loop1 /sys/class/block/loop2 /sys/class/block/loop3 /sys/class/block/loop4 /sys/class/block/loop5 /sys/class/block/loop6 /sys/class/block/loop7 /sys/class/block/sr0 /sys/class/block/vda /sys/class/block/vda1] 
DEBU[0000] ignoring loop or ram device: loop0           
DEBU[0000] ignoring loop or ram device: loop1           
DEBU[0000] ignoring loop or ram device: loop2           
DEBU[0000] ignoring loop or ram device: loop3           
DEBU[0000] ignoring loop or ram device: loop4           
DEBU[0000] ignoring loop or ram device: loop5           
DEBU[0000] ignoring loop or ram device: loop6           
DEBU[0000] ignoring loop or ram device: loop7           
DEBU[0000] checking device: /dev/sr0                    
DEBU[0000] initDisk(): start                            
DEBU[0000] initDisk(): block device                     
DEBU[0000] initDisk(): logical block size 2048, physical block size 4096 
DEBU[0000] trying fat32                                 
DEBU[0000] fat32 failed: blocksize for FAT32 must be either 512 bytes or 0, not 2048 
DEBU[0000] trying iso9660 with physical block size 4096 
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/linuxkit/linuxkit/pkg/metadata/vendor/github.com/diskfs/go-diskfs/filesystem/iso9660.parseDirEntry(0xa60458, 0x0, 0x0, 0xc0000d8000, 0x1c000, 0x0, 0x0)
        /go/src/github.com/linuxkit/linuxkit/pkg/metadata/vendor/github.com/diskfs/go-diskfs/filesystem/iso9660/directoryentry.go:281 +0x789
github.com/linuxkit/linuxkit/pkg/metadata/vendor/github.com/diskfs/go-diskfs/filesystem/iso9660.Read(0x818500, 0xc00000e048, 0x5c000, 0x0, 0x1000, 0x1, 0x8122e0, 0xc0000450c0)
        /go/src/github.com/linuxkit/linuxkit/pkg/metadata/vendor/github.com/diskfs/go-diskfs/filesystem/iso9660/iso9660.go:223 +0x965
github.com/linuxkit/linuxkit/pkg/metadata/vendor/github.com/diskfs/go-diskfs/disk.(*Disk).GetFilesystem(0xc0000765a0, 0x0, 0x0, 0xc0000765a0, 0x0, 0x0)
        /go/src/github.com/linuxkit/linuxkit/pkg/metadata/vendor/github.com/diskfs/go-diskfs/disk/disk.go:227 +0x241
main.FindCIs(0xc0000481e0, 0x7a75a4, 0x1f)
        /go/src/github.com/linuxkit/linuxkit/pkg/metadata/provider_cdrom.go:85 +0x4a0
main.ListCDROMs(0x750dc0, 0xc00000c2a0, 0x4)
        /go/src/github.com/linuxkit/linuxkit/pkg/metadata/provider_cdrom.go:45 +0xf8
main.main()
        /go/src/github.com/linuxkit/linuxkit/pkg/metadata/main.go:105 +0x1795

Describe the results you expected:

metadata to be able to read cloud-config from a cdrom

Additional information you deem important (e.g. issue happens only occasionally):

blkid /dev/sr0: /dev/sr0: BLOCK_SIZE="2048" UUID="2020-10-13-11-17-22-64" LABEL="cidata" TYPE="iso9660"

virsh vol-dumpxml:

<volume type='file'>
  <name>name.iso</name>
  <key>redacted.iso</key>
  <source>
  </source>
  <capacity unit='bytes'>376832</capacity>
  <allocation unit='bytes'>376832</allocation>
  <physical unit='bytes'>376832</physical>
  <target>
    <path>redacted.iso</path>
    <format type='iso'/>
    <permissions>
      <mode>0644</mode>
      <owner>1000</owner>
      <group>1001</group>
      <label>system_u:object_r:virt_content_t:s0</label>
    </permissions>
    <timestamps>
      <atime>1602577058.070323377</atime>
      <mtime>1602577043.271064392</mtime>
      <ctime>1602577044.112135943</ctime>
    </timestamps>
  </target>
</volume>

virsh dumpxml domain:

...
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source file='redacted.iso'/>
      <backingStore/>
      <target dev='hdd' bus='ide'/>
      <readonly/>
      <alias name='ide0-1-1'/>
      <address type='drive' controller='0' bus='1' target='0' unit='1'/>
    </disk>
...