qapi: Rename visitor parameter @variants to @alternatives

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>
6 files changed