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>
