qed: Use bdrv_co_* for coroutine_fns

All functions that are marked coroutine_fn can directly call the
bdrv_co_* version of functions instead of going through the wrapper.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Manos Pitsidianakis <el13635@mail.ntua.gr>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>