domain: expose newly introduced hypercalls as XENFEAT

domain: expose newly introduced hypercalls as XENFEAT

XENFEAT_runstate_phys_area is exposed to all architectures, while
XENFEAT_vcpu_time_phys_area is currently only implemented for x86, and hence
the feature flag is also only exposed on x86.

Additionally add dummy guards with TODOs in the respective hypercall
implementations, to signal the intention to control the availability of those
hypercalls on a guest-by-guest basis from the toolstack.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>