vring: Better error handling if num is too large

To be more consistent inside this function.

Signed-off-by: Fam Zheng <famz@redhat.com>
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>