Fix potential crash when saving guests

Fix potential crash when saving guests

The qemudDomainSaveFlag method will call EndJob on the 'vm'
object it is passed in. This can result in the 'vm' object
being free'd if the last reference is removed. Thus no caller
of 'qemudDomainSaveFlag' must *ever* reference 'vm' again
upon return.

Unfortunately qemudDomainSave and qemuDomainManagedSave
both call 'virDomainObjUnlock', which can result in a
crash. This is non-deterministic since it involves a race
with the monitor I/O thread.

Fix this by making qemudDomainSaveFlag responsible for
calling virDomainObjUnlock instead.

* src/qemu/qemu_driver.c: Fix potential use after free
  when saving guests