vfio/mdev: consolidate all the description sysfs into the core code

Every driver just emits a string, simply add a method to the mdev_driver
to return it and provide a standard sysfs show function.

Remove the now unused types_attrs field in struct mdev_driver and the
support code for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
Link: https://lore.kernel.org/r/20220923092652.100656-14-hch@lst.de


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>