util/guest-random: Fix size arg to tail memcpy

We know that in the body of this if statement i is less than len, so
we really should be copying len - i bytes not i - len bytes.

Fix this typo.

Fixes: 8d8404f1 ("util: Add qemu_guest_getrandom and associated routines")
Signed-off-by: Mark Nelson <mdnelson8@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210709120600.11080-1-mdnelson8@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

util/guest-random.c

@@ -38,7 +38,7 @@ static int glib_random_bytes(void *buf, size_t len)
     }
     if (i < len) {
         x = g_rand_int(rand);
-        __builtin_memcpy(buf + i, &x, i - len);
+        __builtin_memcpy(buf + i, &x, len - i);
     }
     return 0;
 }