virtio: Enable indirect descriptor for vblk and vscsi

With indirect descriptor, we can queue more buffers in the queue.
Indirect descriptor helps block device by making the large request does
not consume the entire ring and making the queue depth deeper. Indirect
descriptor does not help net device because it makes the queue longer so
it adds latency. The tests show that indirect descriptor makes blk
faster and there is no real measurable degradation on net. Also the
indirect will turn on only when we are short of descriptors.

This patch only enables indirect descriptor for vblk and vscsi. vnet is
not enabled.

1) vblk
Before: 340MB/s
After:  350MB/s

2) vscsi
Before: 320MB/s
After:  410MB/s

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>