XenAPI: clean up old snapshots before create new

Before taking a new snapshot, we now inspect the VDI chain to check for
old snapshots.

Those old snapshots can be left behind from resize, migrates, and
snapshots that get interrupted if you kill the nova-compute process.

If you leave these snapshots behind, the hypervisor disk will get too
full, as VDI chains grow much bigger than you would expect. In addition
snapshots start to grow in size, causing issues copying the snapshot
into storage.

Closes-Bug: #1371072

Change-Id: Ice3741213438c7b4a7fda1e3e0a6dd7faca0fd04
Merge "XenAPI: clean up old snapshots before create new"