Prevent VSD deletion when part of the storage volume

[iczerjones]

Today it is possible to forcibly delete an attached VSD, even if it is incorporated into the LVM storage volume. This could lead to data loss and should be accounted for in documentation and possibly programmatically.

Tasks:

• Update documentation to call out potential pitfall of deleting an in-use VSD
• Investigate using the guest agent to detect mounted / used volumes and prevent deletion

Note:
The qemu-guest-agent is preinstalled in Turnkey by default.