[libvirt] RFC: turn cpu 'host-model' into 'custom' for active config on domain start

[Nikolay Shirokovskiy]

Hi, everyone.
  
  Cpu mode 'host-model' is not convinient for migration in current form.
Consider migration from less capable host to more capable one - it is possible.
But then migration backwards is impossible despite the fact that qemu process
on destination is running with the same model and features as on source because
migration does not update 'host-model' cpu upon migration on destination. The
problem is that on migrating back instead of taking current cpu config
'host-model' is expanded again and gain extra features of destination and
migration failed by source (new destination) side.
  
  What if we change cpu for active config from 'host-model' into 'custom' in
the starting process? Then this issue will be fixed. It is quite reasonable 
from my POV - 'host-model' is effectively about taking active domain cpu from host on
start, after that active domain cpu is on its own. Persistent config stays
'host-model' so migrating of active domain and restarting it gains extra
features of new host as before.

Nikolay