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.

shell-completion/bash/systemd-vmspawn

@@ -38,6 +38,7 @@ _systemd_vmspawn() {
         [CONSOLE]='--console'
         [ARG]='--cpus --ram --vsock-cid -M --machine --uuid --private-users --background --set-credential --load-credential'
         [IMAGE_FORMAT]='--image-format'
+        [IMAGE_DISK_TYPE]='--image-disk-type'
     )
 
     _init_completion || return
@@ -59,6 +60,8 @@ _systemd_vmspawn() {
         comps='interactive native gui read-only headless'
     elif __contains_word "$prev" ${OPTS[IMAGE_FORMAT]}; then
         comps='raw qcow2'
+    elif __contains_word "$prev" ${OPTS[IMAGE_DISK_TYPE]}; then
+        comps='virtio-blk virtio-scsi nvme'
     elif __contains_word "$prev" ${OPTS[ARG]}; then
         comps=''
     else