CA-94160: Use allowed_operations system to block/allow VDI.forget.

We now block HA statefile and redo_log VDIs from being forgotten while
HA is enabled.

Forgetting VDIs with plugged VBDs is already blocked, but this patch
also adds forget to VDI.allowed_operations if it is allowed.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1093 from johnelse/ca-94160-clearwater

CLEARWATER: CA-94160: Use allowed_operations system to block/allow VDI.forget.