Do not allow snapshots of read-only disks

[jmpesp]

No description provided.

[leftwo]

Fix for: #9855

[david-crespo]

Doesn't need to be in this PR if it'll delay this getting merged, but there should be an integration test for this.

[jmpesp]

Doesn't need to be in this PR if it'll delay this getting merged, but there should be an integration test for this.

Pushed - the original push was at midnight so I deferred writing a test, but you're right that this should have one.

[leftwo]

I think in the limit, we may want to allow snapshots of read-only disks. If the only way to export a volume is via a snapshot, then it seems unsporting to require users to keep around the source snapshot for a read only disk in case they decide they want to export it. Or, worse, not know they needed to and now it's too late.

But, for now, our underlying implementation does not support it, so best we turn it off.