xen/domctl: Introduce XEN_DOMCTL_CDF_vpci flag

xen/domctl: Introduce XEN_DOMCTL_CDF_vpci flag

Introduce XEN_DOMCTL_CDF_vpci flag to enable VPCI support in XEN.
Reject the use of this new flag for x86 as VPCI is not supported for
DOMU guests for x86.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
[stefano: drop _XEN_DOMCTL_CDF_vpci]
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>