KVM: selftests: Standardise layout of GIC frames

It would appear that all of the selftests are using the same exact
layout for the GIC frames. Fold this back into the library
implementation to avoid defining magic values all over the selftests.

This is an extension of Colton's change, ripping out parameterization of
from the library internals in addition to the public interfaces.

Co-developed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-15-oliver.upton@linux.dev


Signed-off-by: Marc Zyngier <maz@kernel.org>