qapi/commands: Simplify command registry generation

qapi/commands: Simplify command registry generation

QAPISchemaGenCommandVisitor.visit_command() needs to generate the
marshalling function into the current module, and also generate its
registration into the ./init system module.  The latter is done
somewhat awkwardly: .__init__() creates a QAPIGenCCode that will not
be written out, each .visit_command() adds its registration to it, and
.visit_end() copies its contents into the ./init module it creates.

Instead provide the means to temporarily switch to another module.
Create the ./init module in .visit_begin(), and generate its initial
part.  Add registrations to it in .visit_command().  Finish it in
.visit_end().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210201193747.2169670-14-jsnow@redhat.com>