qemu: Use correct host model for updating guest cpu
    
    When a user requested a domain XML description with
    VIR_DOMAIN_XML_UPDATE_CPU flag, libvirt would use the host CPU
    definition from host capabilities rather than the one which will
    actually be used once the domain is started.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1481309
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>