xen: pvhvm: allow user to request no emulated device unplug

this allows the user to disable pvhvm and revert to emulated devices
in case of a system misconfiguration (e.g. initramfs with only
emulated drivers in it).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>
diff --git a/include/xen/platform_pci.h b/include/xen/platform_pci.h
index ce9d671..123b775 100644
--- a/include/xen/platform_pci.h
+++ b/include/xen/platform_pci.h
@@ -21,6 +21,7 @@
 #define XEN_UNPLUG_AUX_IDE_DISKS 4
 #define XEN_UNPLUG_ALL 7
 #define XEN_UNPLUG_IGNORE 8
+#define XEN_UNPLUG_NEVER 16
 
 static inline int xen_must_unplug_nics(void) {
 #if (defined(CONFIG_XEN_NETDEV_FRONTEND) || \