There were many cases were the check was missing:
- changing default_netvm
- resetting netvm to default value
- loading already broken qubes.xml
Since it was possible to create broken qubes.xml using legal calls, do
not reject loading such file, instead break the loop(s) by setting netvm
to None when loop is detected. This will be also useful if still not all
places are covered...
Place the check in default_netvm setter. Skip it during qubes.xml loading
(when events_enabled=False), but still keep it in setter, to _validate_ the
value before any property-* event got fired.
1. It is unclear yet whether dispvm_netvm will be implemented in core3, but
probably not.
2. Remove tests for setting memory/cpu above host resouces - rejecting
those values at property set time would break backup restore on some
machines (when migrating from bigger to smaller system).
