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>

hw/vfio/calxeda-xgmac.c

@@ -20,6 +20,7 @@ static void calxeda_xgmac_realize(DeviceState *dev, Error **errp)
     VFIOCalxedaXgmacDeviceClass *k = VFIO_CALXEDA_XGMAC_DEVICE_GET_CLASS(dev);
 
     vdev->compat = g_strdup("calxeda,hb-xgmac");
+    vdev->num_compat = 1;
 
     k->parent_realize(dev, errp);
 }