block: Use blk_co_ioctl() for all BB level ioctls

All read/write functions already have a single coroutine-based function
on the BlockBackend level through which all requests go (no matter what
API style the external caller used) and which passes the requests down
to the block node level.

This patch exports a bdrv_co_ioctl() function and uses it to extend this
mode of operation to ioctls.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/block/block.h

@@ -318,6 +318,7 @@ void bdrv_aio_cancel(BlockAIOCB *acb);
 void bdrv_aio_cancel_async(BlockAIOCB *acb);
 
 /* sg packet commands */
+int bdrv_co_ioctl(BlockDriverState *bs, int req, void *buf);
 int bdrv_ioctl(BlockDriverState *bs, unsigned long int req, void *buf);
 BlockAIOCB *bdrv_aio_ioctl(BlockDriverState *bs,
         unsigned long int req, void *buf,