vfio-mdev/samples: make mdev_fops const and static

Make this const as it is only passed to a const argument of the function
mdev_register_device. Make it static as it is not referenced in any
other file.

Structure found using Coccinelle and changes done by hand.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>