fix(instances): a migrated VM's source definition is always removed

Migration left the VM defined on the source host unless "Delete
original" was ticked (VIR_MIGRATE_UNDEFINE_SOURCE). With the shared
storage migration needs, that definition points at the disks the
destination VM is running on: the VM showed on both hosts and could be
started (or autostarted) a second time against the same disks; only a
configured lock manager stops that.

moveto now always passes VIR_MIGRATE_UNDEFINE_SOURCE with
VIR_MIGRATE_PERSIST_DEST. The option is gone from the form, the view,
the API (MigrateSerializer.xml_del) and the clone auto-migration. It
removes the definition, not the disk files.

instances/utils.py

@@ -156,7 +156,6 @@ def migrate_instance(
     user,
     live=False,
     unsafe=False,
-    xml_del=False,
     offline=False,
     autoconverge=False,
     compress=False,
@@ -188,7 +187,6 @@ def migrate_instance(
                     instance.name,
                     live,
                     unsafe,
-                    xml_del,
                     offline,
                     autoconverge,
                     compress,