xen: factor out allocation of a single rx entry
xen: factor out allocation of a single rx entry

I'll need this code later to refill the buffer, so factor this out

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@cloudius-systems.com>

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