qdev: Connect busses with their parent devices

This makes SysBus part of the root hierarchy and all busses children of
their respective parent DeviceState.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>