selftests: KVM: Refactor psci_test to make it amenable to new tests

Split up the current test into several helpers that will be useful to
subsequent test cases added to the PSCI test suite.

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/20220504032446.4133305-12-oupton@google.com