KVM: selftests: Multiplex return code and fd in __kvm_create_device()

Multiplex the return value and fd (on success) in __kvm_create_device()
to mimic common library helpers that return file descriptors, e.g. open().

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