qemu-nbd: Close inherited stderr

Close inherited stderr of the parent if fork_process is false.
Otherwise no one will close it. (introduced by e6df58a5)

This only affected 'qemu-nbd -c /dev/nbd0'.

Signed-off-by: Raphael Pour <raphael.pour@hetzner.com>
Message-Id: <d8ddc993-9816-836e-a3de-c6edab9d9c49@hetzner.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: Enhance commit message]
Signed-off-by: Eric Blake <eblake@redhat.com>