vfio/platform: Make the vfio-platform device non-abstract

vfio/platform: Make the vfio-platform device non-abstract

Up to now the vfio-platform device has been abstract and could not be
instantiated.  The integration of a new vfio platform device required
creating a dummy derived device which only set the compatible string.

Following the few vfio-platform device integrations we have seen the
actual requested adaptation happens on device tree node creation
(sysbus-fdt).

Hence remove the abstract setting, and read the list of compatible
values from sysfs if not set by a derived device.

Update the amd-xgbe and calxeda-xgmac drivers to fill in the number of
compatible values, as there can now be more than one.

Note that sysbus-fdt does not support the instantiation of the
vfio-platform device yet.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
[geert: Rebase, set user_creatable=true, use compatible values in sysfs
	instead of user-supplied manufacturer/model options, reword]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>