vfio: selftests: Add helper to set/override a vf_token

vfio: selftests: Add helper to set/override a vf_token

Add a helper function, vfio_device_set_vf_token(), to set or override a
vf_token. Not only at init, but a vf_token can also be set via the
VFIO_DEVICE_FEATURE ioctl, by setting the
VFIO_DEVICE_FEATURE_PCI_VF_TOKEN flag. Hence, add an API to utilize this
functionality from the test code. The subsequent commit will use this to
test the functionality of this method to set the vf_token.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Vipin Sharma <vipinsh@google.com>
Tested-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20260505212838.1698034-7-rananta@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>