QMP: Add qmp command for blockdev-group-snapshot-sync

QMP: Add qmp command for blockdev-group-snapshot-sync

This adds the QMP command for blockdev-group-snapshot-sync. It
takes an array in as the input, for the argument devlist.  The
array consists of the following elements:

    + device:        device to snapshot. e.g. "ide-hd0", "virtio0"
    + snapshot-file: path & file for the snapshot image. e.g. "/tmp/file.img"
    + format:        snapshot format. e.g., "qcow2". Optional

There is no HMP equivalent for the command.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Acked-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>