selftests: KVM: Create helper for making SMCCC calls

The PSCI and PV stolen time tests both need to make SMCCC calls within
the guest. Create a helper for making SMCCC calls and rework the
existing tests to use the library function.

Signed-off-by: Oliver Upton <oupton@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220409184549.1681189-11-oupton@google.com