vhost: remove avail_event arg from vhost_update_avail_event()

In vhost_update_avail_event() we never used the `avail_event` argument,
since its introduction in commit 2723feaa

 ("vhost: set log when
updating used flags or avail event").

Let's remove it to clean up the code.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20220113141134.186773-1-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>