Reversing qdict_array_split(), qdict_flatten() should flatten QLists as
well by interpreting them as QDicts where every entry's key is its
index.

This allows bringing QDicts with QLists from QMP commands to the same
form as they would be given as command-line options, thereby allowing
them to be parsed the same way.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>