target-i386: cpu: make -cpu host/check/enforce code KVM-specific
Rationale:
* "-cpu host" is available only when using KVM
* The current implementation of -cpu check/enforce
(check_features_against_host()) makes sense only when using KVM.
So this makes the functions check_features_against_host() and
cpu_x86_fill_host() KVM-specific, document them as such, and rename them
to kvm_check_features_against_host() and kvm_cpu_fill_host().