domain: migrate: Use UNDEFINE_SOURCE

Leaving a VM defined on the source is dangerous: if they are sharing
storage, and aren't using virtlockd, then they might be inadvertently
started in two places at once and trash a disk image.

Some people might still need the option to turn this off, so maybe we
will revive it. Not sure though

virtManager/domain.py

@@ -1499,6 +1499,7 @@ class vmmDomain(vmmLibvirtObject):
         flags = 0
         flags |= libvirt.VIR_MIGRATE_LIVE
         flags |= libvirt.VIR_MIGRATE_PERSIST_DEST
+        flags |= libvirt.VIR_MIGRATE_UNDEFINE_SOURCE
 
         if secure:
             flags |= libvirt.VIR_MIGRATE_PEER2PEER