openvzRegister: Check for error return

A recent change to openvz_driver.c caused Coverity to make additional
comparisons and find that the openvzRegister() was not checking the
status of virRegisterDriver() call like other callers and thus generated
a CHECKED_RETURN condition

src/openvz/openvz_driver.c

@@ -2239,6 +2239,7 @@ static virDriver openvzDriver = {
 };
 
 int openvzRegister(void) {
-    virRegisterDriver(&openvzDriver);
+    if (virRegisterDriver(&openvzDriver) < 0)
+        return -1;
     return 0;
 }