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>
(cherry picked from commit 7a608f562ebd91e811ed0b725e528c894e4f19c4)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>