tap: add interface to get device fd

Will be used by vhost to attach/detach to backend.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

net/tap.c

@@ -270,6 +270,13 @@ static void tap_poll(VLANClientState *nc, bool enable)
     tap_write_poll(s, enable);
 }
 
+int tap_get_fd(VLANClientState *nc)
+{
+    TAPState *s = DO_UPCAST(TAPState, nc, nc);
+    assert(nc->info->type == NET_CLIENT_TYPE_TAP);
+    return s->fd;
+}
+
 /* fd support */
 
 static NetClientInfo net_tap_info = {

net/tap.h

@@ -48,4 +48,6 @@ int tap_probe_vnet_hdr(int fd);
 int tap_probe_has_ufo(int fd);
 void tap_fd_set_offload(int fd, int csum, int tso4, int tso6, int ecn, int ufo);
 
+int tap_get_fd(VLANClientState *vc);
+
 #endif /* QEMU_NET_TAP_H */