qlit: Tighten QLit dict vs QDict comparison

We check that all members of the QLit dictionary are also in the
QDict.  We neglect to check the other direction.

Comparing the number of members suffices, because QDict can't
contain duplicate members, and putting duplicates in a QLit is a
programming error.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170825105913.4060-12-marcandre.lureau@redhat.com>
[Commit message improved]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
