Switch most 'supported' handling to use virTristateBool, so eventually
we can handle the ABSENT state.

For now the XML formatter treats ABSENT the same as FALSE, so there's
no functional output change. This will be addressed in later patches
Acked-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>