qapi: Move conditional code from QAPISchemaVariants to its subtypes

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>
1 file changed