qemu: Optimize CPU check='partial' for usable CPUs

qemu: Optimize CPU check='partial' for usable CPUs

Ideally check='partial' would check exactly the features QEMU would want
to enable when asked for a specific CPU model (and features). But there
is no way we could ask QEMU how a specific CPU would look like. So we
use our definition from CPU map, which may slightly differ as QEMU adds
or removes features from CPU models, and thus we may end up checking
features which QEMU would not enable while missing some required ones.

We can do better in specific cases, though. If a CPU definition uses
only a model and disabled features (or none at all), we already know
whether QEMU can enable all features required by the CPU model as that's
what we use to set usable='yes' attribute in the list of available CPU
models in domain capbilities XML. So when a usable CPU model is
requested without asking for additional features (disabling features is
fine) we can avoid our possible inaccurate check using our CPU map.

For backward compatibility we only consider usable models. If a
specified model is not usable, we still check it the old way and even
let QEMU start it (and disable some features) in case our definition
lacks some features compared to QEMU.

Fixes: https://gitlab.com/libvirt/libvirt/-/issues/608
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>