qdev: fix hotplug when no -device is specified

The peripheral[-anon] containers are initialized lazily but since they sit on
sysbus, they can not be created after realize.  This was causing an abort() to
occur during hotplug if no -device option was used.

This was spotted by qemu-test::device-add.sh

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
