hw: Fix return value check for bdrv_read, bdrv_write

hw: Fix return value check for bdrv_read, bdrv_write

Those functions return -errno in case of an error.
The old code would typically only detect EPERM (1) errors.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>