Support Real-Time VMs

[LeoBras]

It would be great to be able to run a real-time VM using bootc, taking into account the availability of rt-pretuned-bootc images.

For that a few configurations would be needed:

• Selecting the number of vcpus for the created VM
• Pinning vcpu -> physical cpus
• A way of adding / editing the libvirt XML for the VM. so other tunning can be done

[cgwalters]

A way of adding / editing the libvirt XML for the VM. so other tunning can be done

This strongly relates to virt-manager/virt-manager#739

[lougovsk]

We would like to leverage podman-bootc to spin up pre-tuned RT VMs. This implies that the guest has already some expectations on CPU topology, expecting particular number of isolated vCPUs available and so on. So it is slightly different to the standard situation where a VM starts and then RT being configured. So I was actually thinking if we can leverage LABELs in Containerfile to provide VM expectations to the VMM.