Make libvirt driver.listinstances return defined

Currently the Libvirt driver list_instances doesn't return
defined instances. Those instances required during host initialization
for blueprint rebuild-for-ha

DocImpact

Change-Id: Ifb8f703daa4962f4570c53c75c3d66bc5e5132e7
Co-authored-by: Oshrit Feder <oshritf@il.ibm.com>

nova/tests/fakelibvirt.py

@@ -839,6 +839,9 @@ class Connection(object):
         nwfilter = NWFilter(self, xml)
         self._add_filter(nwfilter)
 
+    def listDefinedDomains(self):
+        return []
+
 
 def openReadOnly(uri):
     return Connection(uri, readonly=True)