vhost: support event index

Support the new event index feature. When acked,
utilize it to reduce the # of interrupts sent to the guest.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --cc drivers/vhost/net.c
Simple merge

diff --cc drivers/vhost/test.c
Simple merge

diff --cc drivers/vhost/vhost.c
Simple merge

diff --cc drivers/vhost/vhost.h
Simple merge