tests: Fix and update for netstress_kill_guest.py script

Update netperf link and find netperf in shared/deps by default.
Also support download netperf from networks.

Add docstring and error.context for this test case.
Put netperf to /var/tmp by default in host and guest.
Also make it configurable.

Signed-off-by: Feng Yang <fyang@redhat.com>