multiboot: Use signed type for negative error numbers

In mb_mod_length a return value is stored that is negative in error case. With
an unsigned type the check goes wrong.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/pc.c

@@ -634,6 +634,7 @@ static int load_multiboot(void *fw_cfg,
                 printf("WARNING: Too many modules loaded, aborting.\n");
                 break;
             }
+
             next_initrd = strchr(initrd_filename, ',');
             if (next_initrd)
                 *next_initrd = '\0';