Fix virDomainGetXMLDesc cache settings output

If a special cache strategy for a disk has been specified in a domain
definition, but no driverName has been set, virDomainGetXMLDesc would not
include the <driver> tag at all.

* src/conf/domain_conf.c: make sure any <driver> tag setting is
  serialized if set.

src/conf/domain_conf.c

@@ -4718,8 +4718,10 @@ virDomainDiskDefFormat(virBufferPtr buf,
                       "    <disk type='%s' device='%s'>\n",
                       type, device);
 
-    if (def->driverName) {
-        virBufferVSprintf(buf, "      <driver name='%s'", def->driverName);
+    if (def->driverName || def->driverType || def->cachemode) {
+        virBufferVSprintf(buf, "      <driver");
+        if (def->driverName)
+            virBufferVSprintf(buf, " name='%s'", def->driverName);
         if (def->driverType)
             virBufferVSprintf(buf, " type='%s'", def->driverType);
         if (def->cachemode)