qapi: Simplify how qmp_dispatch() gets the request ID

qapi: Simplify how qmp_dispatch() gets the request ID

We convert the request object to a QDict twice: first in
qmp_dispatch() to get the request ID, and then again in
qmp_dispatch_check_obj(), which converts to QDict, then checks and
returns it.  We can't get the request ID from the latter, because it's
null when the qdict flunks the checks.

Move the checked conversion to QDict from qmp_dispatch_check_obj() to
qmp_dispatch(), and drop the duplicate there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200317115459.31821-24-armbru@redhat.com>