During detach don't return error if VM is not found

[awels]

What this PR does / why we need it: When detaching a volume from a VM, the VM might not be found if it was deleted for whatever reason. At this point we can assume the volume is no longer hotplugged, and thus the detach succeeded by default.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #83

Special notes for your reviewer:

Release note:

BugFix: volumes attached to nodes that where removed where unable to get detached