The monitor command is:
snapshot_blkdev <device> [snapshot-file] [format]

Default format is qcow2. For now snapshots without a snapshot-file, eg
internal snapshots, are not supported.
Signed-off-by: NJes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>