nbd: Drop unexpected data for NBD_OPT_LIST When requesting the list of exports, no data should be sent. If data is sent, the NBD server should not just inform the client of the invalid request, but also drop the data. Signed-off-by: Max Reitz <mreitz@redhat.com> Message-Id: <1424887718-10800-22-git-send-email-mreitz@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>