qapi: Simplify non-error testing in test-qmp-*

By using &error_abort, we can avoid a local err variable in
situations where we expect success.  It also has the nice
effect that if the test breaks, the error message from
error_abort tends to be nicer than that of g_assert().

This patch has an additional bonus of fixing several call sites that
were passing &err to two different functions without checking it in
between.  In general that is unsafe practice; because if the first
function sets an error, the second function could abort() if it tries to
set a different error. We got away with it because we were asserting
that err was NULL through the entire chain, but switching to
&error_abort avoids the questionable practice up front.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1446791754-23823-7-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
