interrupt: Allow not providing a bottom half thread for easy_register

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
virtio-net: Drop tx_gc_thread thread

We do tx queue gc on the tx path if there is not enough space. The tx
queue gc thread is not a must.

Dropping it saves us a running thread and saves a thread wakeup on every
interrupt.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>