virsh: return correct value from cmdDomIfGetLink

Reported by Alex Jia <ajia@redhat.com>. Function cmdDomIfGetLink did not
set a success return value on success path.

Signed-off-by: Alex Jia<ajia@redhat.com>

tools/virsh.c

@@ -1568,6 +1568,7 @@ hit:
     /* attribute not found */
     vshPrint(ctl, "%s default", iface);
 
+    ret = true;
 cleanup:
     xmlXPathFreeObject(obj);
     xmlXPathFreeContext(ctxt);