Provide the thread ID to handle_event()
Provide the thread ID to handle_event()

By adding a "thread_id" parameter to handle_event(), the backend crate
can now indicate to the backend implementation which thread triggered
the processing of some events.

This is applied to vhost-user-net backend and allows for simplifying a
lot the code since each thread is identical.

Signed-off-by: Sebastien Boeuf <90b401417360b1924ef3f520ddee932686110f85@intel.com>

rust-vmm/vhost