So far, when CPUID data were converted into CPU model and features, the
features can only be added to the model. As a result, when a guest asked
for something like "qemu64,-svm" it would get a qemu32 plus a bunch of
additional features instead.

This patch adds support for removing feature from the base model.
Selection algorithm remains the same: the best CPU model is the model
which requires lowest number of features to be added/removed from it.