This adds tests for live snapshots, both through the single
snapshot command, and the transaction group snapshot command.

The snapshots are done through the QMP interface, using the
following commands for snapshots:

Single snapshot:
{ 'execute': 'blockdev-snapshot-sync', 'arguments':
             { 'device': 'virtio0', 'snapshot-file':'...',
               'format': 'qcow2' } }"

Group snapshot:
{ 'execute': 'transaction', 'arguments':
              {'actions': [
                  { 'type': 'blockdev-snapshot-sync', 'data' :
                    { 'device': 'virtio0', 'snapshot-file': '...' } },
                  { 'type': 'blockdev-snapshot-sync', 'data' :
                    { 'device': 'virtio1', 'snapshot-file': '...' } } ]
             } }

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>