KVM: selftests: Add test for SMCCC filter

Add a selftest for the SMCCC filter, ensuring basic UAPI constraints
(e.g. reserved ranges, non-overlapping ranges) are upheld. Additionally,
test that the DENIED and FWD_TO_USER work as intended.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230404154050.2270077-14-oliver.upton@linux.dev