i386: Add FEAT_8000_0008_EBX CPUID feature word

Add the new feature word and the "ibpb" feature flag.

Based on a patch by Paolo Bonzini.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20180109154519.25634-5-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>