Hosts and VMs internal refactor.

Hosts and VMs internal refactor.

Main changes:
- Netzones contains their hosts.
- Hosts saves their VMs.
- Remove "global" Engine::hosts_.

Possible mapped impacts:
- Hosts destruction order: it was global in the past, now it depends on
the netzones. However, no test affected

Other changes:
- Move code from VirtualMachine interface to VirtualMachineImpl
- Add a HostImpl::create_vm to create VMs: similar to create_host for
NetZoneImpl