qom: Create system containers explicitly

qom: Create system containers explicitly

Always explicitly create QEMU system containers upfront.

Root containers will be created when trying to fetch the root object the
1st time.  They are:

  /objects
  /chardevs
  /backend

Machine sub-containers will be created only until machine is being
initialized.  They are:

  /machine/unattached
  /machine/peripheral
  /machine/peripheral-anon

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20241121192202.4155849-8-peterx@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>