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. Now, you can specify devices for stack-volumes
through:

    VOLUME_BACKING_DEVICE

In case you are using CINDER_MULTI_LVM_BACKEND, you can use

    VOLUME_BACKING_DEVICE2

as well.

Xenserver:
Should you whish to use a 10 gig disk backend for your cinder volumes,
specify:

    XEN_XVDB_SIZE_GB=10
    VOLUME_BACKING_DEVICE=/dev/xvdb

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
Merge "xenapi: separate disk for cinder volumes"