vfio-mdev: move the mtty usage documentation

Move the documentation on how to use mtty to samples/vfio-mdev/README.rst
as it is in no way related to the vfio API.  This matches how the bpf
and pktgen samples are documented.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230110091009.474427-4-hch@lst.de


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