lxc: report correct error

Clang noticed a dead assignment, which turned out to be the use
of the wrong variable.  rc starts life as -1, and is only ever
assigned to 0 just before a successful cleanup.

* src/lxc/lxc_driver.c (lxcSetupInterfaces): Don't call
virReportSystemError(-1).

src/lxc/lxc_driver.c

@@ -1100,10 +1100,9 @@ static int lxcSetupInterfaces(virConnectPtr conn,
         }
 
         if ((ret = brAddInterface(brctl, bridge, parentVeth)) != 0) {
-            virReportSystemError(rc,
+            virReportSystemError(ret,
                                  _("Failed to add %s device to %s"),
                                  parentVeth, bridge);
-            rc = -1;
             goto error_exit;
         }