selftests: KVM: Test SYSTEM_SUSPEND PSCI call

Assert that the vCPU exits to userspace with KVM_SYSTEM_EVENT_SUSPEND if
the guest calls PSCI SYSTEM_SUSPEND. Additionally, guarantee that the
SMC32 and SMC64 flavors of this call are discoverable with the
PSCI_FEATURES call.

Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220504032446.4133305-13-oupton@google.com