Add reason to capabilities

[daviboy]

DCM is making changes to tell users why they can't perform a particular action on a resource. However, many of the Dasein capabilities return just true/false, and I don't think provide a way to get at why eg VirtualMachineCapabilities canStop().

Please provide a way to understand why...

[maksimov]

I suggest adding a new method String VirtualMachineCapabilities#whyNot()

😏