block: directly invoke .bdrv_aio_*() in bdrv_co_io_em()

We will unify block layer request processing across sync, aio, and
coroutines and this means a .bdrv_co_*() emulation function should not
call back into the public interface.  There's no need here, just call
.bdrv_aio_*() directly.

The gory details: bdrv_co_io_em() cannot call back into the public
bdrv_aio_*() interface since that will be handled using coroutines,
which causes us to call into bdrv_co_io_em() again in an infinite loop
:).

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
