qapi: drop the sentinel in enum array

Now that all usages have been converted to user lookup helpers.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170822132255.23945-14-marcandre.lureau@redhat.com>
[Rebased, superfluous local variable dropped, missing
check-qom-proplist.c update added]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503564371-26090-17-git-send-email-armbru@redhat.com>
