qemu: migration: Actually error out on unsupported migration flag

The code reported that a migration flag is unsupported but didn't jump
to the error label. Probably an oversight in commit f88af9dc that
introduced the flag checking.

src/qemu/qemu_migration.c

@@ -1236,6 +1236,7 @@ qemuMigrationCookieXMLParse(qemuMigrationCookiePtr mig,
                            _("Unsupported migration cookie feature %s"),
                            str);
             VIR_FREE(str);
+            goto error;
         }
         VIR_FREE(str);
     }