virtio_pmem: Add freeze/restore callbacks

Add basic freeze/restore PM callbacks to support hibernation (S4):
- On freeze, delete vq and quiesce the device to prepare for
  snapshotting.
- On restore, re-init vq and mark DRIVER_OK.

Signed-off-by: Philip Chen <philipchen@chromium.org>
Message-Id: <20240815004617.2325269-1-philipchen@chromium.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>