x86/xen: add CONFIG_XEN_PV to Kconfig

All code to support Xen PV will get under this new option. For the
beginning, check for it in the common code.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>