sample/vfio-mdev: Change return type to vm_fault_t

convert mbochs_region_vm_fault and mbochs_dmabuf_vm_fault
to return vm_fault_t type.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>