[RFE] Support indirect descriptors feature

[cheese]

Indirect descriptors[1][2] feature dramatically improves throughput on advanced block device.

Upstream Linux accepted this feature long time ago, in both blkback and blkfront. And late Linux distributions already come with xen blkfront driver that support this feature.

[1] https://xenproject.org/2013/08/07/indirect-descriptors-for-xen-pv-disks/
[2] https://lwn.net/Articles/544614/