KVM: selftests: Require vCPU output array when creating VM with vCPUs

Require the caller of __vm_create_with_vcpus() to provide a non-NULL
array of vCPUs now that all callers do so.  It's extremely unlikely a
test will have a legitimate use case for creating a VM with vCPUs without
wanting to do something with those vCPUs, and if there is such a use case,
requiring that one-off test to provide a dummy array is a minor
annoyance.

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