conf: format/parse <vmcoreinfo> as tristate
    
    <features><vmcoreinfo/> is a bare boolean XML property. We don't really
    use this format anymore and instead prefer tristate <X state=on|off/>
    since it's required for modeling on/off/default. If for example future
    qemu started enabling vmcoreinfo by default we wouldn't have any way
    for the user to turn this off.
    
    Convert it to tristate. For writing XML this is semanticly the same,
    <vmcoreinfo/> is processed as <vmcoreinfo state='on'/>.
    
    For apps reading guest XML this is technically an API change,
    as they might misinterpret <vmcoreinfo state='off'/>, however this
    has only been present in libvirt since 3.10.0 and I don't think any
    apps are dependent on this yet
    
    Reviewed-by: John Ferlan <jferlan@redhat.com>
    Signed-off-by: Cole Robinson <crobinso@redhat.com>