Fix loose ends of GET /vm/config post-restore

[alindima]

This issue is a follow up of https://github.com/firecracker-microvm/firecracker/issues/2783. The bulk of the issue was solved in this PR: https://github.com/firecracker-microvm/firecracker/pull/2806

There are still a couple of loose ends that could be fixed in order to get the best user experience:

• net devices MAC addresses are reported as all zeros if left unconfigured. View https://github.com/firecracker-microvm/firecracker/pull/2806#discussion_r793271297
• the machine_config.smt and machine_config.cpu_templates and boot_source values are lost (we could persist them in the VmState).

[dianpopa]

#3121 fixed the network part of this issue. We still have some loose ends to tie (i.e cpu_template, smt and boot-source)