block: drop .bdrv_read()/.bdrv_write() emulation

There is no need to emulate .bdrv_read()/.bdrv_write() since these
interfaces are only called if aio and coroutine interfaces are not
present.  All valid BlockDrivers must implement either sync, aio, or
coroutine interfaces.

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