qapi: Rename QAPISchemaObjectType.variants to .branches

A previous commit narrowed the type of QAPISchemaObjectType.variants
from QAPISchemaVariants to QAPISchemaBranches.  Rename it to
.branches.

Same for .__init__() parameter @variants.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
5 files changed