Introduce CONFIG_XEN_PVHVM compile option

This patch introduce a CONFIG_XEN_PVHVM compile time option to
enable/disable Xen PV on HVM support.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
diff --git a/arch/x86/xen/platform-pci-unplug.c b/arch/x86/xen/platform-pci-unplug.c
index 2f7f3fb..554c002 100644
--- a/arch/x86/xen/platform-pci-unplug.c
+++ b/arch/x86/xen/platform-pci-unplug.c
@@ -32,6 +32,7 @@
 /* store the value of xen_emul_unplug after the unplug is done */
 int xen_platform_pci_unplug;
 EXPORT_SYMBOL_GPL(xen_platform_pci_unplug);
+#ifdef CONFIG_XEN_PVHVM
 static int xen_emul_unplug;
 
 static int __init check_platform_magic(void)
@@ -133,3 +134,4 @@
 	return 0;
 }
 early_param("xen_emul_unplug", parse_xen_emul_unplug);
+#endif