This function returns the host device tree blob from sysfs
(/proc/device-tree). It uses a recursive function inspired
from dtc read_fstree.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>