xenapi: Fix session tests leaking state
xenapi: Fix session tests leaking state

The current tests are setting the `errno` on the `socket.error` *class* which
leaks across tests. This causes the `test_virt_driver.py` tests to fail if run
after the `test_session` tests (which the gate does not do).

The solution is to set the `errno` attribute test on the *instance* so that no
leakage occurs.

Change-Id: I9570523e717db3d436649c0ec396ad1b8152a1c4

JioCloud/nova_test_latest,yosshy/nova,apporc/nova,openstack/nova,takeshineshiro/nova,yatinkumbhare/openstack-nova,belmiromoreira/nova,fnordahl/nova,akash1808/nova,rajalokan/nova,tealover/nova,tudorvio/nova,TwinkleChawla/nova,eonpatapon/nova,Francis-Liu/animated-broccoli,mahak/nova,mahak/nova,iuliat/nova,rajalokan/nova,openstack/nova,rahulunair/nova,Yusuke1987/openstack_template,LoHChina/nova,klmitch/nova,whitepages/nova,tudorvio/nova,scripnichenko/nova,j-carpentier/nova,fnordahl/nova,dawnpower/nova,yosshy/nova,zzicewind/nova,iuliat/nova,j-carpentier/nova,bgxavier/nova,scripnichenko/nova,tangfeixiong/nova,ted-gould/nova,isyippee/nova,yatinkumbhare/openstack-nova,adelina-t/nova,vmturbo/nova,NeCTAR-RC/nova,edulramirez/nova,zzicewind/nova,kimjaejoong/nova,CEG-FYP-OpenStack/scheduler,devendermishrajio/nova_test_latest,mikalstill/nova,bigswitch/nova,cernops/nova,devendermishrajio/nova_test_latest,kimjaejoong/nova,hanlind/nova,devendermishrajio/nova,CloudServer/nova,vmturbo/nova,mahak/nova,watonyweng/nova,BeyondTheClouds/nova,shail2810/nova,JioCloud/nova_test_latest,isyippee/nova,raildo/nova,apporc/nova,cernops/nova,mmnelemane/nova,joker946/nova,devendermishrajio/nova,mikalstill/nova,cernops/nova,noironetworks/nova,blueboxgroup/nova,bigswitch/nova,alexandrucoman/vbox-nova-driver,Francis-Liu/animated-broccoli,Juniper/nova,sebrandon1/nova,Stavitsky/nova,barnsnake351/nova,thomasem/nova,hanlind/nova,Stavitsky/nova,ruslanloman/nova,ruslanloman/nova,vmturbo/nova,alaski/nova,BeyondTheClouds/nova,Juniper/nova,JioCloud/nova,rahulunair/nova,cloudbase/nova,MountainWei/nova,blueboxgroup/nova,shail2810/nova,zaina/nova,varunarya10/nova_test_latest,ted-gould/nova,alexandrucoman/vbox-nova-driver,MountainWei/nova,dims/nova,JioCloud/nova,petrutlucian94/nova,akash1808/nova_test_latest,zhimin711/nova,nikesh-mahalka/nova,mandeepdhami/nova,edulramirez/nova,gooddata/openstack-nova,mikalstill/nova,sebrandon1/nova,Juniper/nova,rajalokan/nova,alvarolopez/nova,dawnpower/nova,CCI-MOC/nova,gooddata/openstack-nova,akash1808/nova,alaski/nova,vmturbo/nova,zhimin711/nova,takeshineshiro/nova,tealover/nova,watonyweng/nova,openstack/nova,raildo/nova,mmnelemane/nova,noironetworks/nova,thomasem/nova,gooddata/openstack-nova,klmitch/nova,JianyuWang/nova,CloudServer/nova,klmitch/nova,felixma/nova,nikesh-mahalka/nova,eonpatapon/nova,LoHChina/nova,phenoxim/nova,BeyondTheClouds/nova,mandeepdhami/nova,Tehsmash/nova,Tehsmash/nova,rajalokan/nova,petrutlucian94/nova,rahulunair/nova,cyx1231st/nova,Juniper/nova,jianghuaw/nova,bgxavier/nova,double12gzh/nova,jeffrey4l/nova,gooddata/openstack-nova,alvarolopez/nova,hanlind/nova,JianyuWang/nova,dims/nova,jeffrey4l/nova,Yusuke1987/openstack_template,klmitch/nova,jianghuaw/nova,CCI-MOC/nova,TwinkleChawla/nova,tangfeixiong/nova,varunarya10/nova_test_latest,felixma/nova,CEG-FYP-OpenStack/scheduler,jianghuaw/nova,NeCTAR-RC/nova,adelina-t/nova,double12gzh/nova,phenoxim/nova,cyx1231st/nova,whitepages/nova,jianghuaw/nova,joker946/nova,zaina/nova,cloudbase/nova,akash1808/nova_test_latest,sebrandon1/nova,cloudbase/nova,barnsnake351/nova,belmiromoreira/nova