QubesException class is used with meaningful messages and should be ok
to use it directly as error message. For other exceptions, still use
full traceback (most likely a bug somewhere, not user error).
FixesQubesOS/qubes-issues#3610
This way we don't need separate admin.vm.Clone call, which is tricky to
handler properly with policy.
A VM may not have access to all the properties and other metadata, so
add ignore_errors argument, for best-effort approach (copy what is
possible). In any case, failure of cloning VM data fails the whole
operation.
When operation fails, VM is removed.
While at it, allow to specify alternative VM class - this allows
morphing one VM into another (for example AppVM -> StandaloneVM).
Adjust qvm-clone tool and tests accordingly.
QubesOS/qubes-issues#2622
