qemu-nbd: Do not close stderr

We kept old_stderr specifically so we could keep emitting error message
on stderr.  However, qemu_daemon() closes stderr.  Therefore, we need to
dup() stderr to old_stderr before invoking qemu_daemon().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190508211820.17851-4-mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>