devtools: disable automatic probing in null testing

devtools: disable automatic probing in null testing

The script test-null.sh is supposed to do a quick and simple
run of testpmd with null PMD only, for sanity check.
As it is not supposed to test probing of any other PMD,
physical device probing is switched to whitelist mode
by using a fake PCI address (0:0.0).
It will also help to keep memory usage stable across platforms.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: David Marchand <david.marchand@redhat.com>