vhost: fix stored last used index

vhost: fix stored last used index

The optimization introduced by
commit d18db8049c7c ("vhost: read last used index once")
didn't account for the fact that vhost_flush_enqueue_shadow_packed
increments the last_used_idx.
For this reason, store last_used_idx after the potential call to
vhost_flush_enqueue_shadow_packed.

Bugzilla ID: 699
Fixes: d18db8049c7c ("vhost: read last used index once")

Signed-off-by: Balazs Nemeth <bnemeth@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Wei Ling <weix.ling@intel.com>