XenAPI: Use SCREEN_LOGDIR rather than assuming a static value
XenAPI: Use SCREEN_LOGDIR rather than assuming a static value

This value was defined in Citrix's install-devstack-xen script, so
only worked for those using that script.

Change-Id: Iab63389f41760865f2b67f6dccd57d774e889905

nawawi/openstack,pacinete/devstack,wenhuizhang/devstack,upadhyay-prashant/devstack,CloudServer/devstack,bswartz/devstack,ozamiatin/devstack,costingalan/devstack,LoHChina/devstack,parul24/devstack,parul24/devstack,affo/devstack,avvocatodemarchis/devstack,costingalan/devstack,wenhuizhang/devstack,nuxeh/devstack,vishnugonela/devstack,openstack-dev/devstack,Mahito/devstack,mssumanth/devstack,olivierlemasle/devstack,upadhyay-prashant/devstack,thomasem/devstack,BSJAIN92/OpenStack,mmasaki/devstack,promptworks/devstack,eharney/devstack,bljgaurav/openstack-test,bq-xiao/devstack,prithivm/branch01,bigswitch/devstack,affo/devstack,nuxeh/devstack,ozamiatin/devstack,flavio-fernandes/devstack,CloudServer/devstack,noironetworks/devstack,mc2014/devstack,bq-xiao/devstack,pczerkas/devstack,sc68cal/devstack,j4/devstack,pczerkas/devstack,mc2014/devstack,richliu/devstack,sequenceiq/devstack,mmasaki/devstack,pacinete/devstack,prithivm/branch01,vsham20/devstack,eharney/devstack,liuquansheng47/devstack,atulpatil301/devstack,NaokiMiyata/devstack,promptworks/devstack,dirkmueller/devstack,kplus/devstack-moonshot,rushiagr/devstack,avvocatodemarchis/devstack,herrsechs/devstack,andrewrothstein/devstack,liuquansheng47/devstack,thomasem/devstack,andrewrothstein/devstack,herrsechs/devstack,group-policy/devstack,bswartz/devstack,samgoon/devstack,flavio-fernandes/devstack,sequenceiq/devstack,j4/devstack,vishnugonela/devstack,atulpatil301/devstack,olivierlemasle/devstack,rushiagr/devstack,mssumanth/devstack,dtroyer/devstack,HPCHub/devstack,richliu/devstack,openstack-dev/devstack,HPCHub/devstack,pombredanne/devstack,BSJAIN92/OpenStack,sc68cal/devstack,dirkmueller/devstack,varunarya10/devstack,kplus/devstack-moonshot,neerja28/Devstack_GlusterFS,neerja28/Devstack_GlusterFS,group-policy/devstack,TwinkleChawla/devstack,dtroyer/devstack,TwinkleChawla/devstack,bigswitch/devstack,varunarya10/devstack,samgoon/devstack,NaokiMiyata/devstack,noironetworks/devstack,pombredanne/devstack,vsham20/devstack,nawawi/openstack,Mahito/devstack,bljgaurav/openstack-test,LoHChina/devstack