Do not check iterable objects before for loop
Do not check iterable objects before for loop

get_VIFs, get_VBDs will return set

For more details see the XenAPI docs:
https://docs.citrix.com/content/dam/docs/en-us/xenserver/xenserver-62/xenenterpriseapi.pdf


Change-Id: Ic3f0e2eb18d5d6408c60979383465575e0a99d05

openstack/ceilometer,openstack/ceilometer