Don't crash without a security driver

"virsh dominfo <vm>" crashes if there's no primary security driver set
since we only intialize the secmodel.model and secmodel.doi if we have
one. Attached patch checks for securityPrimaryDriver instead of
securityDriver since the later is always set in qemudSecurityInit().

Closes: http://bugs.debian.org/574359

src/qemu/qemu_driver.c

@@ -4979,7 +4979,7 @@ static int qemudNodeGetSecurityModel(virConnectPtr conn,
     int ret = 0;
 
     qemuDriverLock(driver);
-    if (!driver->securityDriver) {
+    if (!driver->securityPrimaryDriver) {
         memset(secmodel, 0, sizeof (*secmodel));
         goto cleanup;
     }