The QMP handler qmp_object_add() and the implementation of --object in
qemu-storage-daemon can share most of the code. Currently,
qemu-storage-daemon calls qmp_object_add(), but this is not correct
because different visitors need to be used.

As a first step towards a fix, make qmp_object_add() a wrapper around a
new function user_creatable_add_dict() that can get an additional
parameter. The handling of "props" is only required for compatibility
and not required for the qemu-storage-daemon command line, so it stays
in qmp_object_add().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>