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>

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    - HVM PIRQs are disabled by default.
    - Reduce IOMMU setup time for hardware domain.
    - Allow HVM/PVH domains to map foreign pages.
+   - Declare PVH dom0 supported with caveats.
  - xl/libxl configures vkb=[] for HVM domains with priority over vkb_device.
  - Increase the maximum number of CPUs Xen can be built for from 4095 to
    16383.