Usability is returned in unusable field, so not need to clobber entire
AR. Callers have to know how to deal with unusable segments already
since if emulate_invalid_guest_state=true AR is not zeroed.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>