utils.process: Use actual duration instead of timeout value

The interrupted message might be slightly misleading as there are
several timeouts before declaring the process interrupted and the wait
might not be issued immediatelly so let's use the (time - start_time)
value instead of timeout value.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/utils/process.py

@@ -822,7 +822,8 @@ class SubProcess(object):
                     the specified timeout.
         """
         def nuke_myself():
-            self.result.interrupted = "timeout after %ss" % timeout
+            self.result.interrupted = ("timeout after %ss"
+                                       % (time.time() - self.start_time))
             try:
                 kill_process_tree(self.get_pid(), sig, timeout=1)
             except Exception: