In order to be consistent, we shouldn't advertise a GICv3 when none
is actually usable by the guest.

Wipe the feature when these conditions apply, and allow the field
to be written from userspace.

This now allows us to rewrite the kvm_has_gicv3 helper() in terms
of kvm_has_feat(), given that it is always evaluated at runtime.
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240827152517.3909653-6-maz@kernel.orgSigned-off-by: Marc Zyngier <maz@kernel.org>