Extract the pid field from the trace record and print it.

Change the trace record tuple from:
  (event_num, timestamp, arg1, ..., arg6)
to:
  (event_num, timestamp, pid, arg1, ..., arg6)

Trace event methods now support 3 prototypes:
1. <event-name>(arg1, arg2, arg3)
2. <event-name>(timestamp, arg1, arg2, arg3)
3. <event-name>(timestamp, pid, arg1, arg2, arg3)

Existing script continue to work without changes, they only know about
prototypes 1 and 2.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>