xenapi: separate disk for cinder volumes

Some kernels have lockup issues while serving cinder volumes from
file-loopback-lvm kind of cinder setup. This patch provides a way to
create an additional drive to the OpenStack VM, and use that to store
cinder volumes, thus eliminating the lockup issue. It will help when
testing XenServer. Citrix is using this approach on its internal CI
system to run tests against OpenStack. This is a workaround for this
bug:

  https://bugs.launchpad.net/cinder/+bug/1023755

Related to blueprint xenapi-devstack-cleanup

Change-Id: Iee633d2704185bfbf9234882654c47b850fa168a