block/qed: use buffer-based io

Move to _co_ versions of io functions qed_read_table() and
qed_write_table(), as we use qemu_co_mutex_unlock()
anyway.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>