block/io: bdrv_co_do_copy_on_readv: lazy allocation

Allocate bounce_buffer only if it is really needed. Also, sub-optimize
allocation size (why not?).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190604161514.262241-7-vsementsov@virtuozzo.com
Message-Id: <20190604161514.262241-7-vsementsov@virtuozzo.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>