qjson: Fix qobject_from_json() & friends for multiple values

qjson: Fix qobject_from_json() & friends for multiple values

qobject_from_json() & friends use the consume_json() callback to
receive either a value or an error from the parser.

When they are fed a string that contains more than either one JSON
value or one JSON syntax error, consume_json() gets called multiple
times.

When the last call receives a value, qobject_from_json() returns that
value.  Any other values are leaked.

When any call receives an error, qobject_from_json() sets the first
error received.  Any other errors are thrown away.

When values follow errors, qobject_from_json() returns both a value
and sets an error.  That's bad.  Impact:

* block.c's parse_json_protocol() ignores and leaks the value.  It's
  used to to parse pseudo-filenames starting with "json:".  The
  pseudo-filenames can come from the user or from image meta-data such
  as a QCOW2 image's backing file name.

* vl.c's parse_display_qapi() ignores and leaks the error.  It's used
  to parse the argument of command line option -display.

* vl.c's main() case QEMU_OPTION_blockdev ignores the error and leaves
  it in @err.  main() will then pass a pointer to a non-null Error *
  to net_init_clients(), which is forbidden.  It can lead to assertion
  failure or other misbehavior.

* check-qjson.c's multiple_values() demonstrates the badness.

* The other callers are not affected since they only pass strings with
  exactly one JSON value or, in the case of negative tests, one
  error.

The impact on the _nofail() functions is relatively harmless.  They
abort when any call receives an error.  Else they return the last
value, and leak the others, if any.

Fix consume_json() as follows.  On the first call, save value and
error as before.  On subsequent calls, if any, don't save them.  If
the first call saved a value, the next call, if any, replaces the
value by an "Expecting at most one JSON value" error.  Take care not
to leak values or errors that aren't saved.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-44-armbru@redhat.com>