x86/pvh: declare PVH dom0 supported with caveats

x86/pvh: declare PVH dom0 supported with caveats

PVH dom0 is functionally very similar to PVH domU except for the domain
builder and the added set of hypercalls available to it.

The main concern with declaring it "Supported" is the lack of some features
when compared to classic PV dom0, hence switch it's status to supported with
caveats.  List the known missing features, there might be more features missing
or not working as expected apart from the ones listed.

Note there's some (limited) PVH dom0 testing on both osstest and gitlab.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>