vhost: support indirect descriptor in non-mergeable Rx

Linux virtio-net kernel driver uses indirect descriptors when
mergeable buffers are not used.

This patch adds its support, fixing the use of indirect
descriptors with these guests.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>