tests: Don't crash when creating the config object fails

As observed when building in a chroot and QEMU_USER doesn't exist

tests/qemuargv2xmltest.c

@@ -128,6 +128,9 @@ mymain(void)
     int ret = 0;
 
     driver.config = virQEMUDriverConfigNew(false);
+    if (driver.config == NULL)
+        return EXIT_FAILURE;
+
     if ((driver.caps = testQemuCapsInit()) == NULL)
         return EXIT_FAILURE;
 

tests/qemuxml2argvtest.c

@@ -501,6 +501,9 @@ mymain(void)
     }
 
     driver.config = virQEMUDriverConfigNew(true);
+    if (driver.config == NULL)
+        return EXIT_FAILURE;
+
     VIR_FREE(driver.config->spiceListen);
     VIR_FREE(driver.config->vncListen);