CA-143836: Pass down correct values of is_a_snapshot/snapshot_of

Signed-off-by: John Else <john.else@citrix.com>
CA-143836: Allow VDI.revert operation for cloneable snapshots

Signed-off-by: John Else <john.else@citrix.com>
CA-143836: During VDI.revert, mark leaf with operation `reverting

This mirrors the way allowed_operations are handled for VM.revert.

Signed-off-by: John Else <john.else@citrix.com>
CA-143836: Add vdi_revert to SR allowed_operations

Signed-off-by: John Else <john.else@citrix.com>
CA-143836: Add VDI.revert test to quicktest

Signed-off-by: John Else <john.else@citrix.com>