KVM: selftests: Add a helper for SMCCC calls with SMC instruction

Build a helper for doing SMCs in selftests by macro-izing the current
HVC implementation and taking the conduit instruction as an argument.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230404154050.2270077-13-oliver.upton@linux.dev