qapi-commands: Don't feed output of mcgen() to mcgen() again

Multiple passes through mcgen() is prone to produce unwanted blank
lines, which we then combat by sprinkling .rstrip() on top.  Just
don't do it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>