KVM: arm/arm64: Clarification and relaxation to ITS save/restore ABI

Clarify what is meant by the save/restore ABI only supporting virtual
physical interrupts.

Relax the requirement of the order that the collection entries are
written in and be clear that there is no particular ordering enforced.

Some cosmetic changes in the capitalization of ID names to align with
the GICv3 manual and remove the empty line in the bottom of the patch.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>