QAPISchemaVariants.check()'s code is almost entirely conditional on
union vs. alternate type.
Move the conditional code to QAPISchemaBranches.check() and
QAPISchemaAlternatives.check(), where the conditions are always
satisfied.
Attribute QAPISchemaVariants.tag_name is now only used by
QAPISchemaBranches. Move it there.
Refactor the three types' .__init__() to make them a bit simpler.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
8152bc7de6