qobject: Replace qobject_from_jsonf() by qobject_from_jsonf_nofail()

qobject: Replace qobject_from_jsonf() by qobject_from_jsonf_nofail()

Commit ab45015a968 "qobject: Let qobject_from_jsonf() fail instead of
abort" fails to accomplish its stated aim: the function can still
abort due to its use of &error_abort.

Its rationale for letting it fail is that all remaining users cope
fine with failure.  Well, they're just fine with aborting, too; it's
what they do on failure.

Simply reverting the broken commit would bring back the unfortunate
asymmetry between qobject_from_jsonf() and qobject_from_jsonv(): one
aborts, the other returns null.  So also rename it to
qobject_from_jsonf_nofail().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-7-armbru@redhat.com>