vhost: reenable indirect descriptors

Patch 808a2b05 [1] fixed overflow of avail entries count,
also fixing indirect descriptors support. Without indirect
descriptors, we usually didn't manage to fill entire queue
- there were always a couple of descriptors left. A couple,
but not enough to form a vhost-scsi descriptor. Indirect
descriptors, however, are 1-descriptor long. They have higher
possibility to fill entire avail queue, and that's why they
used to fail more often. But we can safely reenable them now.

[1] 808a2b05 ("vhost: fix overflow of avail entries count")

Change-Id: Idec8568cdfc1255fb578e5d18f5c476a4c034d2d
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/396805


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>