virtio: change descriptor free list to FIFO instead of LIFO
virtio: change descriptor free list to FIFO instead of LIFO

Based on observation that with packets comprised of multiple fragments
vhost_get_vq_desc() goes higher in CPU profile. Avi suggested that the
current LIFO handling of free descriptors causes contention on cache
lines between seastar on vhost.

Gives 6-10% boost depending on hardware.

xtao/seastar,wildinto/scylla,bowlofstew/seastar,scylladb/scylla,dwdm/seastar,bzero/seastar,joerg84/seastar,bzero/seastar,raphaelsc/scylla,justintung/scylla,flashbuckets/seastar,kjniemi/scylla,gwicke/scylla,phonkee/scylla,capturePointer/scylla,linearregression/seastar,shyamalschandra/seastar,aruanruan/scylla,eklitzke/scylla,bzero/seastar,tempbottle/seastar,gwicke/scylla,tempbottle/scylla,hongliangzhao/seastar,linearregression/scylla,flashbuckets/seastar,scylladb/scylla,mixja/seastar,stamhe/seastar,kjniemi/seastar,norcimo5/seastar,bowlofstew/scylla,glommer/scylla,linearregression/scylla,norcimo5/seastar,cloudius-systems/seastar,wildinto/seastar,avikivity/scylla,dreamsxin/seastar,bowlofstew/seastar,raphaelsc/seastar,syuu1228/seastar,sjperkins/seastar,sjperkins/seastar,leejir/seastar,rluta/scylla,dwdm/seastar,mixja/seastar,linearregression/scylla,xtao/seastar,avikivity/seastar,shyamalschandra/seastar,stamhe/scylla,kjniemi/scylla,acbellini/scylla,wildinto/scylla,scylladb/seastar,tempbottle/seastar,dwdm/scylla,jonathanleang/seastar,scylladb/scylla,glommer/scylla,joerg84/seastar,capturePointer/scylla,victorbriz/scylla,kjniemi/seastar,chunshengster/seastar,dwdm/scylla,kangkot/scylla,raphaelsc/scylla,asias/scylla,kangkot/scylla,printedheart/seastar,bowlofstew/seastar,dwdm/scylla,stamhe/scylla,stamhe/seastar,respu/scylla,bowlofstew/scylla,sjperkins/seastar,guiquanz/scylla,justintung/scylla,joerg84/seastar,avikivity/seastar,scylladb/scylla,jonathanleang/seastar,raphaelsc/scylla,leejir/seastar,shaunstanislaus/scylla,flashbuckets/seastar,acbellini/seastar,senseb/scylla,acbellini/scylla,ducthangho/imdb,printedheart/seastar,koolhazz/seastar,guiquanz/scylla,cloudius-systems/seastar,acbellini/seastar,asias/scylla,stamhe/scylla,avikivity/seastar,tempbottle/seastar,aruanruan/scylla,stamhe/seastar,syuu1228/seastar,shaunstanislaus/scylla,wildinto/seastar,asias/scylla,rentongzhang/scylla,scylladb/seastar,jonathanleang/seastar,gwicke/scylla,ducthangho/imdb,tempbottle/scylla,anzihenry/seastar,victorbriz/scylla,hongliangzhao/seastar,tempbottle/scylla,rluta/scylla,kjniemi/scylla,justintung/scylla,scylladb/scylla-seastar,rentongzhang/scylla,duarten/scylla,avikivity/scylla,linearregression/seastar,raphaelsc/seastar,rentongzhang/scylla,duarten/scylla,koolhazz/seastar,aruanruan/scylla,capturePointer/scylla,duarten/scylla,guiquanz/scylla,kjniemi/seastar,dwdm/seastar,ducthangho/imdb,dreamsxin/seastar,scylladb/seastar,eklitzke/scylla,chunshengster/seastar,respu/scylla,shaunstanislaus/scylla,anzihenry/seastar,linearregression/seastar,slivne/seastar,victorbriz/scylla,koolhazz/seastar,kangkot/scylla,mixja/seastar,printedheart/seastar,avikivity/scylla,norcimo5/seastar,rluta/scylla,slivne/seastar,wildinto/seastar,acbellini/scylla,senseb/scylla,glommer/scylla,acbellini/seastar,shyamalschandra/seastar,wildinto/scylla,leejir/seastar,scylladb/scylla-seastar,respu/scylla,bowlofstew/scylla,chunshengster/seastar,eklitzke/scylla,syuu1228/seastar,dreamsxin/seastar,anzihenry/seastar,phonkee/scylla,cloudius-systems/seastar,slivne/seastar,hongliangzhao/seastar,xtao/seastar,scylladb/scylla-seastar,senseb/scylla,phonkee/scylla,raphaelsc/seastar