Update libvirt config syntax for new version of driver domain patches

Finally accepted patches uses different syntax: <backenddomain name=.../> tag
instead of <source domain=.../>.

core-modules/000QubesVm.py

@@ -1016,7 +1016,7 @@ class QubesVm(object):
                    "      <mac address='{mac}'/>\n" \
                    "      <ip address='{ip}'/>\n" \
                    "      <script path='vif-route-qubes'/>\n" \
-                   "      <source domain='{backend}'/>\n" \
+                   "      <backenddomain name='{backend}'/>\n" \
                    "    </interface>\n"
         return template.format(ip=ip, mac=mac, backend=backend)
 

core/storage/xen.py

@@ -63,7 +63,7 @@ class QubesXenVmStorage(QubesVmStorage):
         if not rw:
             params += "      <readonly/>\n"
         if domain:
-            params += "      <domain name='%s'/>\n" % domain
+            params += "      <backenddomain name='%s'/>\n" % domain
         if script:
             params += "      <script path='%s'/>\n" % script
         return template.format(path=path, vdev=vdev, type=type,