qapi: Tighten qmp_input_end_list()

The only way that qmp_input_pop() will set errp is if a dictionary
was the most recent thing pushed.  Since we don't have any
push(struct)/pop(list) or push(list)/pop(struct) mismatches (such
a mismatch is a programming bug), we therefore cannot set errp
inside qmp_input_end_list().  Make this obvious by
using &error_abort.  A later patch will then remove the errp
parameter of qmp_input_pop(), but that will first require the
larger task of splitting visit_end_struct().

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1454075341-13658-23-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>