block: Always compile virtio-blk dataplane

Dataplane doesn't depend on linux-aio any more, so we don't need the
compiling condition now.

Configure options are kept but just print a message.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1410329871-28885-4-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>