util: add some debug log to virNetDevGetMaster

This makes it easier to see why libvirt has decided it must re-attach
a tap device to its bridge.

Signed-off-by: Laine Stump <laine@laine.org>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>

src/util/virnetdev.c

@@ -977,6 +977,7 @@ virNetDevGetMaster(const char *ifname, char **master)
             goto cleanup;
     }
 
+    VIR_DEBUG("IFLA_MASTER for %s is %s", ifname, *master ? *master : "(none)");
     ret = 0;
  cleanup:
     VIR_FREE(nlData);