xen: make pvscsi frontend dependant on xenbus frontend

The pvscsi frontend driver requires the xenbus frontend driver. Reflect
this in Kconfig.
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

drivers/scsi/Kconfig

@@ -579,6 +579,7 @@ config VMWARE_PVSCSI
 config XEN_SCSI_FRONTEND
 	tristate "XEN SCSI frontend driver"
 	depends on SCSI && XEN
+	select XEN_XENBUS_FRONTEND
 	help
 	  The XEN SCSI frontend driver allows the kernel to access SCSI Devices
 	  within another guest OS (usually Dom0).