vhost: read last used index once

vhost: read last used index once

Instead of calculating the address of a packed descriptor based on the
vq->desc_packed and vq->last_used_idx every time, store that base
address in desc_base. On arm, this saves 176 bytes in code size of
function in which vhost_flush_enqueue_batch_packed gets inlined.

Signed-off-by: Balazs Nemeth <bnemeth@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>