vl: Ensure qapi visitor properly ends struct visit

vl: Ensure qapi visitor properly ends struct visit

Guarantee that visit_end_struct() is called if
visit_start_struct() succeeded.  This matches the behavior of
most other uses of visitors, and is a step towards the possibility
of a future patch that adds and enforces some tighter semantics to
the visitor interface (namely, cleanup of the visitor would no
longer have to mop up as many leftovers from an aborted partial
visit).

The change to code here matches the flow of hmp.c:hmp_object_add();
a later patch will then further simplify the cleanup logic of both
places by refactoring visit_end_struct() to not require a second
local error object.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1454075341-13658-9-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>