Prior to 1143ec5e it was OK to qemu_iovec_from_buf() from aligned-up
buffer to original qiov, as qemu_iovec_from_buf() will stop at qiov end
anyway.

But after 1143ec5e we assume that bdrv_co_do_copy_on_readv works on
part of original qiov, defined by qiov_offset and bytes. So we must not
touch qiov behind qiov_offset+bytes bound. Fix it.

Cc: qemu-stable@nongnu.org # v4.2
Fixes: 1143ec5e
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20200312081949.5350-1-vsementsov@virtuozzo.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>