samples/vfio-mdev/mbochs: expand minor range when registering chrdev region

      Actually, total amount of available minor number
for a single major is MINORMARK + 1. So expand
minor range when registering chrdev region.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>