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/drivers/xen/Kconfig b/drivers/xen/Kconfig
index 8f84b10..0a88269 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -64,7 +64,7 @@
 
 config XEN_PLATFORM_PCI
 	tristate "xen platform pci device driver"
-	depends on XEN
+	depends on XEN_PVHVM
 	default m
 	help
 	  Driver for the Xen PCI Platform device: it is responsible for