Prevent mount point collision on validation

**Describe the solution you'd like:**
It is possible to set multiple volumes to be mounted to the same path. We can assume the end-user knows what they are doing, but should be a very straightforward validation.

**Why do you want this feature:**
To prevent unnecessary failures because of user error.

**Anything else you would like to add:**
Validate 2 things:
* Root volume mount point is `/`
* Do not allow multiple volumes to be mounted on the same path

Reference: https://github.com/weaveworks/flintlock/discussions/241#discussioncomment-1630857

The issue does not name files or tests. Start by locating the volume validation entry point used during microVM creation, then inspect existing validation coverage. Done means validation enforces a root volume mounted at `/` and rejects multiple volumes using the same mount path, with tests for both cases.

Written by the indexing model from the issue text.