XenAPI: Increase OpenStack DomU usage again
XenAPI: Increase OpenStack DomU usage again

Devstack has continued to grow in memory requirements and now we cannot
reliably fit in 4GB, with several services being unable to start.  Increase
the minimum for DomU to 6GB to leave room for virtual machines

Change-Id: Idbdfa1f36015b6af347d1ce27eb28baa360af5ef

upadhyay-prashant/devstack,flavio-fernandes/devstack,mc2014/devstack,Mahito/devstack,thomasem/devstack,herrsechs/devstack,j4/devstack,dirkmueller/devstack,pacinete/devstack,HPCHub/devstack,NaokiMiyata/devstack,vishnugonela/devstack,olivierlemasle/devstack,LoHChina/devstack,mmasaki/devstack,TwinkleChawla/devstack,bswartz/devstack,wenhuizhang/devstack,prithivm/branch01,dirkmueller/devstack,noironetworks/devstack,dtroyer/devstack,atulpatil301/devstack,liuquansheng47/devstack,openstack-dev/devstack,eharney/devstack,bigswitch/devstack,samgoon/devstack,nawawi/openstack,j4/devstack,varunarya10/devstack,ozamiatin/devstack,costingalan/devstack,herrsechs/devstack,HPCHub/devstack,olivierlemasle/devstack,costingalan/devstack,pombredanne/devstack,LoHChina/devstack,nawawi/openstack,parul24/devstack,mssumanth/devstack,atulpatil301/devstack,mssumanth/devstack,pombredanne/devstack,eharney/devstack,vsham20/devstack,mmasaki/devstack,mc2014/devstack,upadhyay-prashant/devstack,samgoon/devstack,pczerkas/devstack,CloudServer/devstack,wenhuizhang/devstack,group-policy/devstack,richliu/devstack,prithivm/branch01,parul24/devstack,CloudServer/devstack,avvocatodemarchis/devstack,Mahito/devstack,NaokiMiyata/devstack,bq-xiao/devstack,bigswitch/devstack,bq-xiao/devstack,varunarya10/devstack,bswartz/devstack,flavio-fernandes/devstack,dtroyer/devstack,pczerkas/devstack,thomasem/devstack,pacinete/devstack,vishnugonela/devstack,liuquansheng47/devstack,openstack-dev/devstack,noironetworks/devstack,vsham20/devstack,ozamiatin/devstack,richliu/devstack,TwinkleChawla/devstack,avvocatodemarchis/devstack,group-policy/devstack