domain: backfill listen address to parent <graphics> listen attribute

Prior to 0.9.4, libvirt only supported a single listen, and it had to
be an IP address:

   <graphics listen='1.2.3.4' ..../>

Starting with 0.9.4, a graphics element could have a <listen>
subelement (actually the grammar supports multiples, but all of the
drivers only support a single <listen> per <graphics>), and that
listen element can be of type='address' or type='network'. For
type='address', <listen> also has an attribute called 'address' which
contains the IP address for listening:

    <graphics ....>
      <listen type='address' address='1.2.3.4' .../>
    </graphics>

type can also be "network", and in that case listen will have a
"network" attribute which will contain the name of a libvirt
network:

    <graphics ....>
      <listen type='network' network='testnet' .../>
    </graphics>

At domain start (or migrate) time, libvirt will attempt to
find an IP address associated with that network (e.g. the IP address
of the bridge device used by the network, or the physical device
listed in <forward dev='physdev'/>) and fill in that address in the
status XML:

    <graphics ....>
      <listen type='network' network='testnet' address='1.2.3.4' .../>
    </graphics>

In the case that a <graphics> element has a <listen> subelement of
type='address', that listen subelement's "address" attribute is
backfilled into the parent graphics element's "listen" *attribute* for
backward compatibility (so that a management application unaware of
the separate <listen> element can still learn the listen
address). This backfill should be done with the IP learned from
type='network' as well, and that's what this patch does:

    <graphics listen='1.2.3.4' ....>
      <listen type='network' network='testnet' address='1.2.3.4' .../>
    </graphics>

This is a continuation of the fix for:

   https://bugzilla.redhat.com/show_bug.cgi?id=1191016

src/conf/domain_conf.c

@@ -18915,17 +18915,23 @@ virDomainGraphicsDefFormat(virBufferPtr buf,
         return -1;
     }
 
-    /* find the first listen element of type='address' and duplicate
-    * its address attribute as the listen attribute of
-    * <graphics>. This is done to improve backward compatibility. */
+    /* find the first listen subelement with a valid address and
+    * duplicate its address attribute as the listen attribute of
+    * <graphics>. This is done to improve backward compatibility.
+    */
     for (i = 0; i < def->nListens; i++) {
-        if (virDomainGraphicsListenGetType(def, i)
-            == VIR_DOMAIN_GRAPHICS_LISTEN_TYPE_ADDRESS) {
-            if (flags & VIR_DOMAIN_DEF_FORMAT_MIGRATABLE &&
-                def->listens[i].fromConfig)
-                continue;
-            listenAddr = virDomainGraphicsListenGetAddress(def, i);
-            break;
+        virDomainGraphicsListenType listenType;
+
+        if (flags & VIR_DOMAIN_DEF_FORMAT_MIGRATABLE &&
+            def->listens[i].fromConfig)
+            continue;
+        listenType = virDomainGraphicsListenGetType(def, i);
+
+        if (listenType == VIR_DOMAIN_GRAPHICS_LISTEN_TYPE_ADDRESS ||
+            (listenType == VIR_DOMAIN_GRAPHICS_LISTEN_TYPE_NETWORK &&
+             !(flags & VIR_DOMAIN_DEF_FORMAT_INACTIVE))) {
+            if ((listenAddr = virDomainGraphicsListenGetAddress(def, i)))
+                break;
         }
     }