Support removing features when converting data to CPU

Support removing features when converting data to CPU

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.