xen: XEN_PVHVM depends on PCI

Xen PV on HVM guests require PCI support because they need the
xen-platform-pci driver in order to initialize xenbus.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>