scripts: add render_block_graph function for QEMUMachine

scripts: add render_block_graph function for QEMUMachine

Render block nodes graph with help of graphviz. This new function is
for debugging, so there is no sense to put it into qemu.py as a method
of QEMUMachine. Let's instead put it separately.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20181221170909.25584-3-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>