Very few calls at client side really needs VM class name. So, even in
non-blind mode use just QubesVM class, to avoid strange cases depending
on blind mode being enabled or not. Then, have VM class name in 'klass'
property. If known at object creation time, cache it, otherwise query
qubesd at first access.
This allows to perform actions on objects (VM, storage etc), without
listing them. This is useful when calling VM have minimal permissions
and only selected actions are allowed.
This means that app.domains['some-name'] will not raise KeyError, even
when domain do not exists. But performing actual action (like
vm.start()) will fail in that case.
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
Add qubesd_call(..., payload_stream=...) argument to allow streaming
payload directly from some file/process stdout. This is mainly (only?)
useful for admin.vm.volume.Import, where disk volume raw data is passed
to the service.
