Make dma_bdrv_io available for drivers, and pass an explicit I/O function
instead of hardcoding bdrv_aio_readv/bdrv_aio_writev.  This is required
to implement non-READ/WRITE dma commands in the ide driver, e.g. the
upcoming TRIM support.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>