qapi: Add qobject_to()

qapi: Add qobject_to()

This is a dynamic casting macro that, given a QObject type, returns an
object as that type or NULL if the object is of a different type (or
NULL itself).

The macro uses lower-case letters because:
1. There does not seem to be a hard rule on whether qemu macros have to
   be upper-cased,
2. The current situation in qapi/qmp is inconsistent (compare e.g.
   QINCREF() vs. qdict_put()),
3. qobject_to() will evaluate its @obj parameter only once, thus it is
   generally not important to the caller whether it is a macro or not,
4. I prefer it aesthetically.

The macro parameter order is chosen with typename first for
consistency with other QAPI macros like QAPI_CLONE(), as well as
for legibility (read it as "qobject to" type "applied to" obj).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20180224154033.29559-3-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
[eblake: swap parameter order to list type first, avoid clang ubsan
warning on QOBJECT(NULL) and container_of(NULL,type,base)]
Signed-off-by: Eric Blake <eblake@redhat.com>