TPM emulation only needed (currently) on OSX VMs

[e-minguez]

According to #64 (comment) TPM emulation is only needed if the VM is created on UTM (and UTM included support for TPM on 4.3 so it needs a little bit of research if we need it at all).

[hardys]

@e-minguez I think on Linux we'd need to add configuration to enable an emulated TPM - elemental-register expects access to a TPM.

In the case of UTM I did try the 4.3 TPM support but I couldn't get working - it doesn't just enable the device, it forces secure-boot which prevented my SLEMicro VM from booting.

As you say we need some further research, but the flag I added works around the issue by emulating the TPM in the elemental-register code instead.