Fix return value usage

Fix the error checking to use the return value from brAddTap() instead
of checking the current errno value which might have been changed by
clean up calls inside of brAddTap().

Signed-off-by: Doug Goldstein <cardoe@gentoo.org>

src/qemu/qemu_conf.c

@@ -1689,7 +1689,7 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
                         tapmac,
                         vnet_hdr,
                         &tapfd))) {
-        if (errno == ENOTSUP) {
+        if (err == ENOTSUP) {
             /* In this particular case, give a better diagnostic. */
             qemuReportError(VIR_ERR_INTERNAL_ERROR,
                             _("Failed to add tap interface to bridge. "