qapi: More tests of input arrays

Our testsuite had no coverage of empty arrays, nor of what
happens when the input does not match the expected type.
Useful to have, especially if we start changing the visitor
contracts.

I did not think it worth duplicating these additions to
test-qmp-input-strict; since all strict mode does is add
the ability to reject JSON input that has more keys than
what the visitor expects, yet the additions in this patch
error out earlier than that point regardless of whether
strict mode was requested.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1446791754-23823-11-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>