qapi: change QmpInputVisitor to QSLIST

This saves a lot of memory compared to a statically-sized array,
or at least 24kb could be considered a lot on an Atari ST.
It also makes the code more similar to QmpOutputVisitor.

This removes the limit on the depth of a QObject that can be processed
into a QAPI tree.  This is not a problem because QObjects can be
considered trusted; the text received on the QMP wire is untrusted
input, but the JSON parser already takes pains to limit the QObject tree
it creates.  We don't need the QMP input visitor to limit it again.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1467906798-5312-3-git-send-email-pbonzini@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message typo fixed]
Signed-off-by: Markus Armbruster <armbru@redhat.com>