lxc: free error object to avoid memory leak

Detected by Coverity. Leak introduced in commit 9d201a5c.

* src/lxc/lxc_driver.c: Clean up on failure.
Signed-off-by: NAlex Jia <ajia@redhat.com>

src/lxc/lxc_driver.c

@@ -1857,7 +1857,7 @@ cleanup:
 
     if (err) {
         virSetError(err);
-        virResetError(err);
+        virFreeError(err);
     }
 
     return rc;