Use virtual instead of profile time for vmprofile

Avoids infinite looping due to system call restarts
triggered by SIGPROF handling.

Virtual time doesn't account for time spent in the
kernel handling system calls but Snabb tries to avoid
spending time in system calls.

(cherry picked from commit c4681a6)

src/lj_vmprofile.c

@@ -88,20 +88,20 @@ static void start_timer(int interval)
   struct sigaction sa;
   tm.it_value.tv_sec = tm.it_interval.tv_sec = interval / 1000;
   tm.it_value.tv_usec = tm.it_interval.tv_usec = (interval % 1000) * 1000;
-  setitimer(ITIMER_PROF, &tm, NULL);
+  setitimer(ITIMER_VIRTUAL, &tm, NULL);
   sa.sa_flags = SA_SIGINFO | SA_RESTART;
   sa.sa_sigaction = vmprofile_signal;
   sigemptyset(&sa.sa_mask);
-  sigaction(SIGPROF, &sa, &state.oldsa);
+  sigaction(SIGVTALRM, &sa, &state.oldsa);
 }
 
 static void stop_timer()
 {
   struct itimerval tm;
   tm.it_value.tv_sec = tm.it_interval.tv_sec = 0;
   tm.it_value.tv_usec = tm.it_interval.tv_usec = 0;
-  setitimer(ITIMER_PROF, &tm, NULL);
-  sigaction(SIGPROF, NULL, &state.oldsa);
+  setitimer(ITIMER_VIRTUAL, &tm, NULL);
+  sigaction(SIGVTALRM, NULL, &state.oldsa);
 }
 
 /* -- State that the application can manage via FFI ----------------------- */