A previous commit narrowed the type of .visit_alternate_type()
parameter @variants from QAPISchemaVariants to QAPISchemaAlternatives.
Rename it to @alternatives.
One of them passes @alternatives to helper function
gen_visit_alternate(). Rename its @variants parameter to
@alternatives as well.
Signed-off-by: Markus Armbruster <armbru@redhat.com>