vhost: add unlikely annotations to error path

patch 'break out of polling loop on error' caused
a minor performance regression on my machine: recover
that performance by adding a bunch of unlikely annotations
in the error handling.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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

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