qapi-introspect: Mangle --prefix argument properly for C

qapi-introspect.py --prefix hasn't been used so far, but fix it anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1488544368-30622-7-git-send-email-armbru@redhat.com>
[Commit message improved]
Reviewed-by: Eric Blake <eblake@redhat.com>