qemu-nbd: Fix return value handling of bdrv_open

bdrv_open may return -errno so we have to check
if the return value is '< 0', not '== -1'.
Signed-off-by: NRyota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

qemu-nbd.c

@@ -333,7 +333,7 @@ int main(int argc, char **argv)
     if (bs == NULL)
         return 1;
 
-    if (bdrv_open(bs, argv[optind], flags) == -1)
+    if (bdrv_open(bs, argv[optind], flags) < 0)
         return 1;
 
     fd_size = bs->total_sectors * 512;