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: Ryota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
