qcow2: fix iovec size at qcow2_co_pwritev_compressed

Use bytes as the size would be more exact than s->cluster_size.  Although
qemu_iovec_to_buf() will not allow to go beyond the qiov.

Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>