xen/domctl: Add DOMINFO_hap to xen_domctl_getdomaininfo

xen/domctl: Add DOMINFO_hap to xen_domctl_getdomaininfo

This allows a toolstack to identify whether a running domain is using hardware
assisted paging or not.

The appropriate tests differ by architecture, so introduce
arch_get_domain_info().  ARM unconditionally sets the new flag, while x86
checks with the paging subsystem first.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>