Advertise need for 'python-dpkt' system module

It's needed by TCP-related tracing commands, as of now:

 trace.py list --tcpdump
 trace.py tcpdump

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
trace: fix recommendation given by error message in case dpkt is missing

Since we adveritse use of a system package in README, we should also
recommend the same here.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tests: introduce tracing smoke test

The test verifies basic tracing functionallity:
 * that samples are collected by OSv
 * extraction via `trace extract` succeeds
 * `trace summary` shows expected tracepoint names
 * listing via `trace list` shows tracepoints with arguments
 * network packet parsing works

This test is run as part of standard 'make check'.

Note: this single test can be executed as easily as:

 $ scripts/test.py --test tracing_smoke_test

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>