vmspawn: add --bind-volume= command line option

vmspawn: add --bind-volume= command line option

  systemd-vmspawn --bind-volume=PROVIDER:VOLUME[:CONFIG][:K=V,...]

For each --bind-volume passed at startup, vmspawn calls Acquire() on
the named StorageProvider and attaches the resulting fd to the VM as
an additional drive. The drive is identified by the user-visible name
'<provider>:<volume>' on the bridge — that is also the handle used
later when machinectl unbind-volume detaches drives at runtime
(though boot-time drives like these are NOT removable; that is the
StorageImmutable behaviour added earlier).

The colon grammar is parsed by the shared bind_volume_parse() helper.
The 3rd 'config' field selects the guest device type from the
disk_type_table[] vocabulary (virtio-blk, virtio-scsi, nvme, scsi-cd);
empty defaults to virtio-blk per the TASK grammar.

Wiring lives next to the existing --extra-drive setup: parse_argv()
appends a parsed BindVolume to arg_bind_volumes, and prepare_device_info()
hands the array to vmspawn_bind_volume_prepare_boot() which Acquires
each volume and pushes a DriveInfo onto the existing drives array.
PCIe port assignment (assign_pcie_ports()) and the QMP setup loop pick
them up automatically.

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