sysctl/libxl: choose a sane default for HAP

sysctl/libxl: choose a sane default for HAP

Current libxl code will always enable Hardware Assisted Paging (HAP),
expecting that the hypervisor will fallback to shadow if HAP is not
available. With the changes to DOMCTL_createdomain that's not the case
any longer, and the hypervisor will raise an error if HAP is not
available instead of silently falling back to shadow.

In order to keep the previous functionality report whether HAP is
available or not in XEN_SYSCTL_physinfo, so that the toolstack can
select a sane default if there's no explicit user selection of whether
HAP should be used.

Note that on ARM hardware HAP capability is always reported since it's
a required feature in order to run Xen.

Fixes: d0c0ba7d3de ('x86/hvm/domain: remove the 'hap_enabled' flag')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>