libvirt: Reset can_fallocate in test setUp()
libvirt: Reset can_fallocate in test setUp()

imagebackend.Image stores global data in Image.can_fallocate. This is
normally irrelevant, but can cause the test_prealloc_image tests
to fail non-deterministically depending on test order.

Change-Id: I21e666de66b39130d98c6126a4e0c0b9932d7286

mahak/nova,hanlind/nova,mikalstill/nova,openstack/nova,rajalokan/nova,sebrandon1/nova,vmturbo/nova,openstack/nova,rahulunair/nova,openstack/nova,jianghuaw/nova,jianghuaw/nova,klmitch/nova,Juniper/nova,rajalokan/nova,gooddata/openstack-nova,sebrandon1/nova,klmitch/nova,cloudbase/nova,rahulunair/nova,jianghuaw/nova,mikalstill/nova,Juniper/nova,cloudbase/nova,rajalokan/nova,jianghuaw/nova,vmturbo/nova,vmturbo/nova,klmitch/nova,phenoxim/nova,klmitch/nova,phenoxim/nova,vmturbo/nova,mikalstill/nova,mahak/nova,rahulunair/nova,Juniper/nova,rajalokan/nova,cloudbase/nova,gooddata/openstack-nova,gooddata/openstack-nova,hanlind/nova,alaski/nova,gooddata/openstack-nova,sebrandon1/nova,hanlind/nova,Juniper/nova,mahak/nova,alaski/nova