qemu-storage-daemon: Fix non-string --object properties

After processing the option string with the keyval parser, we get a
QDict that contains only strings. This QDict must be fed to a keyval
visitor which converts the strings into the right data types.

qmp_object_add(), however, uses the normal QObject input visitor, which
expects a QDict where all properties already have the QType that matches
the data type required by the QOM object type.

Change the --object implementation in qemu-storage-daemon so that it
doesn't call qmp_object_add(), but calls user_creatable_add_dict()
directly instead and pass it a new keyval boolean that decides which
visitor must be used.

Reported-by: Coiby Xu <coiby.xu@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>