vmspawn: add disk type selection for root and extra drives (#41301)

vmspawn: add disk type selection for root and extra drives (#41301)

  vmspawn previously hardcoded virtio-blk for all drives. This adds
  --image-disk-type= to select the root disk type (virtio-blk,
  virtio-scsi, or nvme) and allows per-drive overrides via a
  colon-separated prefix on --extra-drive=. The format and disk type
  prefixes can appear in any order since their value sets don't overlap.

  For virtio-scsi, a single shared controller is created with drives
  attached as scsi-hd devices. For nvme, each drive gets its own
controller. Both have serial number length limits (30 and 20 characters
  respectively), so long filenames are replaced with a truncated SHA-256
  hex digest.