Fix up the locking in the snapshot code.

Fix up the locking in the snapshot code.

In particular I was forgetting to take the qemuMonitorPrivatePtr
lock (via qemuDomainObjBeginJob), which would cause problems
if two users tried to access the same domain at the same time.
This patch also fixes a problem where I was forgetting to remove
a transient domain from the list of domains.

Thanks to Stephen Shaw for pointing out the problem and testing
out the initial patch.

Signed-off-by: Chris Lalancette <clalance@redhat.com>