qdev: add children before qdev_init

qdev: add children before qdev_init

We want the composition tree to to be in order by the time we call
qdev_init, so that a single set of the toplevel realize property can
propagate all the way down the composition tree.

This is not the case so far.  Unfortunately, this is incompatible
with calling qdev_init in the constructor wrappers for devices,
so for now we need to unattach some devices that are created through
those wrappers.  This will be fixed by removing qdev_init and instead
setting the toplevel realize property after machine init.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>