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().

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
