selftests: KVM: Use KVM_SET_MP_STATE to power off vCPU in psci_test

Setting a vCPU's MP state to KVM_MP_STATE_STOPPED has the effect of
powering off the vCPU. Rather than using the vCPU init feature flag, use
the KVM_SET_MP_STATE ioctl to power off the target vCPU.

Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220504032446.4133305-11-oupton@google.com