CHANGELOG.md: Mention PCI passthrough for HVM domUs

PCI passthrough is already supported for HVM domUs when dom0 is PVH
on x86. The last related patch on Qemu side was merged after Xen4.20
release. So mention this feature in Xen4.21 entry.

But SR-IOV is not yet supported on PVH dom0, add a note for it.

Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -12,6 +12,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
  - On x86:
    - Option to attempt to fixup p2m page-faults on PVH dom0.
    - Resizable BARs is supported for PVH dom0.
+   - Support PCI passthrough for HVM domUs when dom0 is PVH (note SR-IOV
+     capability usage is not yet supported on PVH dom0).
 
 ### Removed