KVM: selftests: Convert s390x/diag318_test_handler away from VCPU_ID

Convert diag318_test_handler to use vm_create_with_vcpus() and pass around a
'struct kvm_vcpu' object instead of passing around vCPU IDs.  Note, this is
a "functional" change in the sense that the test now creates a vCPU with
vcpu_id==0 instead of vcpu_id==6.  The non-zero VCPU_ID was 100% arbitrary
and added little to no validation coverage.  If testing non-zero vCPU IDs
is desirable for generic tests, that can be done in the future by tweaking
the VM creation helpers.

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