vfio-mdev: fix some error codes in the sample code

This is just sample code.  We forget to set the error codes in a couple
places.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>