KVM: selftests: Convert psci_test away from VCPU_ID

Pass around 'struct kvm_vcpu' objects in psci_test instead of relying on
global VCPU_IDs.  Ideally, the test wouldn't have to manually create
vCPUs and thus care about vCPU IDs, but it's not the end of the world and
avoiding that behavior isn't guaranteed to be a net positive (an attempt
at macro shenanigans did not go very well).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>