lib: Almost eliminate use of virTristateBoolTypeFromString()

lib: Almost eliminate use of virTristateBoolTypeFromString()

There are couple of places where virTristateBoolTypeFromString()
is called. Well, the same result can be achieved by
virXMLPropTristateBool() and on fewer lines.

Note there are couple of places left untouched because those
don't care about error reporting and thus are shorter they way
they are now.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>