qemu: Redundant listen address entry in quest xml

When editing guest's XML (on QEMU), it was possible to add multiple
listen elements into graphics parent element. However QEMU does not
support listening on multiple addresses. Configuration is tested for
multiple 'listen address' and if positive, an error is raised.

https://bugzilla.redhat.com/show_bug.cgi?id=1119212

src/qemu/qemu_process.c

@@ -3950,6 +3950,11 @@ int qemuProcessStart(virConnectPtr conn,
                     goto cleanup;
                 }
                 graphics->listens[0].fromConfig = true;
+            } else if (graphics->nListens > 1) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                               _("QEMU does not support multiple listen "
+                                 "addresses for one graphics device."));
+                goto cleanup;
             }
         }
     }