vfio: Provide a get_region_info op

vfio: Provide a get_region_info op

Instead of hooking the general ioctl op, have the core code directly
decode VFIO_DEVICE_GET_REGION_INFO and call an op just for it.

This is intended to allow mechanical changes to the drivers to pull their
VFIO_DEVICE_GET_REGION_INFO int oa function. Later patches will improve
the function signature to consolidate more code.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/1-v2-2a9e24d62f1b+e10a-vfio_get_region_info_op_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>