conf: properly skip graphics listen element in migratable XML

We should skip <listen type='socket'/> only if the 'socket' path
is specified because if there is no 'socket' path we need to
keep that element in migratable XML.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1366088Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/conf/domain_conf.c

@@ -23134,6 +23134,7 @@ virDomainGraphicsDefFormat(virBufferPtr buf,
              * thus we can generate it in the migrateble XML. */
             if (def->type == VIR_DOMAIN_GRAPHICS_TYPE_VNC &&
                 def->listens[i].type == VIR_DOMAIN_GRAPHICS_LISTEN_TYPE_SOCKET &&
+                def->listens[i].socket &&
                 !def->listens[i].autoGenerated)
                 continue;