Fix up qemu domain save/managed save locking.

Fix up qemu domain save/managed save locking.

The current version of the qemu managed save implementation
is subject to a race where the domain shuts down between
the time that we start the command and the time that we
actually try to do the save.  Close this race by making
qemuDomainSaveFlags() expect both the driver and the passed-in
vm object to be locked before executing.

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