Xen: Attempt to find and cleanup orphaned SR during delete

Before some patches merged to better handle failures to boot an instance
from a volume it was possible to have an attached volume with an SR and
plugged PBD but no VM.  And the destroy method in xenapi short circuits
if no VM is present because everything is looked up from there in order
to be cleaned up.

This adds a check to see if a volume is supposed to be attached during
destroy if no VM is present and attempts to look it up via the SR uuid
and clean it up.

I would like to say that this patch is unnecessary, as it's sort of
hackish, but it's possible that some deployments have instances in this
state and this will allow them to cleanup without manually running
commands on the hypervisor.

Closes-bug: #1367918
Change-Id: I7745af7e22164a96af800ae796bda077e49bf291
Merge "Xen: Attempt to find and cleanup orphaned SR during delete"