util: more verbose error when failing to create macvtap device

Investigation of a problem with creating passthrough macvtap devices
(https://bugzilla.redhat.com/show_bug.cgi?id=1185501) has shown that
this slightly more verbose failure message is useful. In particular,
the mac address can be used to determine the domain. You could also
figure this out by looking at preceding messages in a debug log, but
this gets it in a single place.

src/util/virnetdevmacvlan.c

@@ -107,6 +107,7 @@ virNetDevMacVLanCreate(const char *ifname,
     unsigned int recvbuflen;
     struct nl_msg *nl_msg;
     struct nlattr *linkinfo, *info_data;
+    char macstr[VIR_MAC_STRING_BUFLEN];
 
     if (virNetDevGetIndex(srcdev, &ifindex) < 0)
         return -1;
@@ -177,8 +178,9 @@ virNetDevMacVLanCreate(const char *ifname,
 
         default:
             virReportSystemError(-err->error,
-                                 _("error creating %s type of interface attach to %s"),
-                                 type, srcdev);
+                                 _("error creating %s interface %s@%s (%s)"),
+                                 type, ifname, srcdev,
+                                 virMacAddrFormat(macaddress, macstr));
             goto cleanup;
         }
         break;