sysctl: report existing physcaps on Arm

sysctl: report existing physcaps on Arm

Current physcaps in XEN_SYSCTL_physinfo are only used by x86, albeit
the capabilities themselves are not x86 specific.

This patch adds support for also reporting the current capabilities on
Arm hardware. Note that on Arm PHYSCAP_hvm is always reported, and
setting PHYSCAP_directio has been moved to common code since the same
logic to set it is used by x86 and Arm.

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>