Introduce format string for pid_t

BeOS and Haiku on i386 use long for 32-bit types, including pid_t.
Using %d with pid_t therefore results in a warning.

Unfortunately POSIX:2008 does not define a PRId* string for pid_t.

In some places pid_t was previously casted to long and %ld hardcoded.
The predecessor of this patch added another upcast for the simpletrace
filename but was not applied to date.

Since new uses of pid_t with %d keep creeping in, let's instead define
an OS-dependent format string and use that consistently.

Cc: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Ingo Weinhold <ingo_weinhold@gmx.de>
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

vl.c

@@ -1197,7 +1197,7 @@ void qemu_kill_report(void)
              */
             fputc('\n', stderr);
         } else {
-            fprintf(stderr, " from pid %d\n", shutdown_pid);
+            fprintf(stderr, " from pid " FMT_pid "\n", shutdown_pid);
         }
         shutdown_signal = -1;
     }