linux-phytec-fsl

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>