vdi: Replace bdrv_{read,write}() with bdrv_{pread,pwrite}()

vdi: Replace bdrv_{read,write}() with bdrv_{pread,pwrite}()

There's only a couple of bdrv_read() and bdrv_write() calls left in
the vdi code, and they can be trivially replaced with the byte-based
bdrv_pread() and bdrv_pwrite().

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>