VMware: Disallow snapshot of attached volume

The current implementation perform snapshot of attached volume.
This is invalid because when we take snapshot of the backing, the instance
continues pointing to the readonly delta and tries writing to it.
This patch will disallow snapshot creation of attached volume.

We have similar issue with delete of snapshot, when deleted the instance will
point to an invalid disk file, since that will get merged with its parent.
Hence any writes by the instance will fail with disk not found error.
This patch will ignore delete snapshot of attached volume.

Since we disallow snapshot of attached volume, we disallow linked clone of
attached source volume and also upload of attached volume.

Fixes bug: 1236891

Change-Id: I6a91eac8529836d1fe1de73bf6b28ec098aa7c54
(cherry picked from commit 2451a027bee50c834890f9e049fbaf4bfa911546)
Merge "VMware: Disallow snapshot of attached volume" into milestone-proposed