snapshot: wire up qemu transaction command

snapshot: wire up qemu transaction command

The hardest part about adding transactions is not using the new
monitor command, but undoing the partial changes we made prior
to a failed transaction.

* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateDiskActive): Use
transaction when available.
(qemuDomainSnapshotUndoSingleDiskActive): New function.
(qemuDomainSnapshotCreateSingleDiskActive): Pass through actions.
(qemuDomainSnapshotCreateXML): Adjust caller.