kvm tools: Support virtio indirect buffers

kvm tools: Support virtio indirect buffers

Indirect buffers are ring descriptors which point to more (even more)
descriptors.

This can be used to increase the effective ring capacity, which helps the
guest to batch large requests - very useful for blk devices.

This patch also enables indirect buffers for virtio-net and virtio-blk.

The patch is based on the lguest's code which does the same.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>