qdict: Make qdict_flatten() shallow-clone-friendly

qdict: Make qdict_flatten() shallow-clone-friendly

In its current form, qdict_flatten() removes all entries from nested
QDicts that are moved to the root QDict.  It is completely sufficient to
remove all old entries from the root QDict, however.  If the nested
dicts have a refcount of 1, this will automatically delete them, too.
And if they have a greater refcount, we probably do not want to modify
them in the first place.

The latter observation means that it was currently (in general)
impossible to qdict_flatten() a shallowly cloned dict because that would
empty nested QDicts in the original dict as well.  This patch changes
this, so you can now use qdict_flatten(qdict_shallow_clone(dict)) to get
a flattened copy without disturbing the original.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20180611205203.2624-7-mreitz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>