i386: Rename X86CPU::host_features to X86CPU::max_features

Rename the field and add a small comment to make its purpose
clearer.

Message-Id: <20170119210449.11991-4-ehabkost@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>