[prop4n]

Adds a configurable disk bus and makes the disk resize tolerant of an image
already at the requested size. Both surfaced while running Guix cloud images,
which expect virtio-blk (/dev/vda) rather than the default virtio-scsi (/dev/sda).

Disk bus

disks[].bus, values scsi (default) or virtio:

disks:
  - storage: local-lvm
    size: 20G
    bus: virtio        # guest sees /dev/vda; default scsi -> /dev/sda

• Applies to VirtualMachine (blank and image) and Template.
• A clone inherits the template's bus: the clone path detects the slot
  (virtio0 vs scsi0) from the cloned config instead of assuming scsi0.
• Import, boot order and resize all target the resolved slot.

Resize is "at least this size"

An image already at (or above) the requested size made Proxmox reject the resize
as a shrink, failing creation. size now means "grow to at least this"; a shrink
error is treated as "already big enough" and ignored.

Tests

• diskSlot mapping, bus validation (manifest), bus threaded to template/clone
  spec, shrink-error detection.
• Verified end to end on a live cluster: a virtio template builds and boots a
  Guix image on /dev/vda; a 20G image no longer fails the resize step.

golangci-lint (v2), gofmt, Go + vitest suites green.