guest agent: remove g_strcmp0 usage

g_strcmp0 isn't in all version of glib 2.0, so don't use it to avoid
build breakage on older distros.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qemu-ga.c

@@ -146,7 +146,7 @@ static void ga_log(const gchar *domain, GLogLevelFlags level,
     }
 
     level &= G_LOG_LEVEL_MASK;
-    if (g_strcmp0(domain, "syslog") == 0) {
+    if (domain && strcmp(domain, "syslog") == 0) {
         syslog(LOG_INFO, "%s: %s", level_str, msg);
     } else if (level & s->log_level) {
         g_get_current_time(&time);