vfio: UAPI for migration interface for device state

- Defined MIGRATION region type and sub-type.

- Defined vfio_device_migration_info structure which will be placed at the
  0th offset of migration region to get/set VFIO device related
  information. Defined members of structure and usage on read/write access.

- Defined device states and state transition details.

- Defined sequence to be followed while saving and resuming VFIO device.
Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Reviewed-by: Neo Jia <cjia@nvidia.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>