qmp-input: Clean up stack handling

qmp-input: Clean up stack handling

Management of the top of stack was a bit verbose; creating a
temporary variable and adding some comments makes the existing
code more legible before the next few patches improve things.
No semantic changes other than asserting that we are always
visiting a QObject, and not a NULL value.  In particular, the
check for 'name && qobject_type(qobj) == QTYPE_QDICT)' is a
bit overkill (a dict visit should always have a name); a later
patch revisits that, while this patch is only changing one
layer of indentation due to dropping 'if (qobj)'.

Signed-off-by: Eric Blake <[email protected]>
Message-Id: <1461879932[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>