tap: unbreak -netdev tap,fd=X

The multiqueue patch series broke -netdev tap,fd=X which manifests
as libvirt not being able to start a guest.  This was because it
passed NULL for the netdev name which results in an anonymous netdev
device regardless of what the user specified.

Cc: Jason Wang <jasowang@redhat.com>
Cc: Bruce Rogers <brogers@suse.com>
Reported-by: Bruce Rogers <brogers@suse.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

net/tap.c

@@ -711,7 +711,7 @@ int net_init_tap(const NetClientOptions *opts, const char *name,
 
         vnet_hdr = tap_probe_vnet_hdr(fd);
 
-        if (net_init_tap_one(tap, peer, "tap", NULL, NULL,
+        if (net_init_tap_one(tap, peer, "tap", name, NULL,
                              script, downscript,
                              vhostfdname, vnet_hdr, fd)) {
             return -1;