qdict: Fix memory leak in qdict_do_flatten()

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
qdict: Optimise qdict_do_flatten()

Nested QDicts used to be both entered recursively in order to move their
entries to the target QDict and also be moved themselves to the target
QDict like all other objects. This is harmless because for the top
level, qdict_do_flatten() will encounter the (now empty) QDict for a
second time and then delete it, but at the same time it's obviously
unnecessary overhead. Just delete nested QDicts directly after moving
all of their entries.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>