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>

os-posix.c

@@ -368,7 +368,7 @@ int qemu_create_pidfile(const char *filename)
     if (lockf(fd, F_TLOCK, 0) == -1) {
         return -1;
     }
-    len = snprintf(buffer, sizeof(buffer), "%ld\n", (long)getpid());
+    len = snprintf(buffer, sizeof(buffer), FMT_pid "\n", getpid());
     if (write(fd, buffer, len) != len) {
         return -1;
     }