vmspawn: add nvme disk type support

vmspawn: add nvme disk type support

Extend --image-disk-type= and the --extra-drive= disk type prefix to
support nvme in addition to virtio-blk and virtio-scsi:

  systemd-vmspawn --image-disk-type=nvme --image=image.raw
  systemd-vmspawn --image=image.raw --extra-drive=nvme:data.raw

The NVMe serial number is limited to 20 characters by the NVMe spec.
If the image filename exceeds this, it is hashed with SHA-256 and
truncated to 20 hex characters via the disk_serial() helper introduced
in the previous commit.

Signed-off-by: Christian Brauner <brauner@kernel.org>