When qubesd reject access to property (do not return anything), it may
mean various things - invalid property, permission denied, or even
invalid VM. Handle all of those with exception inheriting from
AttributeError.
This may be useful for things like AttributeError, ValueError etc.
Use error name directly as exception name without much validation, as it
comes from dom0.
It doesn't make sense to send mgmt call to _named_ VM just to ask for
its name. Use value that QubesVM object already have.
This also means we can safely access vm.name, no need to touch any
private attribute.
QubesOS/qubes-issues#853
