hw/vfio/platform: amd-xgbe device

This patch introduces the amd-xgbe VFIO platform device. It
allows the guest to do passthrough on a device exposing an
"amd,xgbe-seattle-v1a" compat string.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
