conf: Introduce hyperv host-model mode

conf: Introduce hyperv host-model mode

So far we have two modes for hyperv features:

1) custom, where users have to enable features explicitly, and
2) passthrough, where hypervisor enables features automagically.

Problem with 'custom' mode is that some features are not plain
on/off switches but expect int/string value. Until very recently,
these were not reported in domcaps. And even if they were it's a
bit cumbersome.

Problem with 'passthrough' mode is that users don't get to see
the expanded list of enlightenments enabled.

Therefore, mimic what we're already doing with CPUs: have
'host-model' which gets expanded at domain startup and is fixed
throughout domain's run.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>