XenAPI: Remove interrupted snapshots

Currently the VDI chain can grow very long when a snapshots happen at
the same time as nova-compute being terminated.

While we now clean up the instance state, the VDI chain is left in a bad
state, it has an extra snapshot that is no longer required.

This change improves that by looking at when we detect a failed
snapshot, we go back and tidy up the VDI chain.

Partial-Bug: #1331440

Change-Id: I9bae82048910d8c45bc2a4093064c1ac68f15750
Merge "XenAPI: Remove interrupted snapshots"