qapi: Rename visitor parameter @variants to @branches

qapi: Rename visitor parameter @variants to @branches

The previous commit narrowed the type of .visit_object_type()
parameter @variants from QAPISchemaVariants to QAPISchemaBranches.
Rename it to @branches.

Same for .visit_object_type_flat().

A few of these pass @branches to helper functions:
QAPISchemaGenRSTVisitor.visit_object_type() to ._nodes_for_members()
and ._nodes_for_variant_when(), and
QAPISchemaGenVisitVisitor.visit_object_type() to
gen_visit_object_members().  Rename the helpers' @variants parameters
to @branches as well.

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