Add QMP bits for snapshot_blkdev command. This is the same as
snapshot_blkdev in the human monitor. The command is synchronous.

In the future async commands and or a break down of the functionality
into multiple commands might be added.

Also change the 'snapshot_file' argument to 'snapshot-file' in
the human monitor, so that it matches QMP.
Signed-off-by: NJes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@gmail.com>