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 is a wrapper to use `$dispvm` target of qrexec call, just like any
other service call in qubesadmin module - using vm.run_service().
When running in dom0, qrexec-client-vm is not available, so DispVM needs
to be created "manually", using appropriate Admin API call
(admin.vm.CreateDisposable).
QubesOS/qubes-issues#2974
Since we use qubes.VMShell service now and send requested command on its
stdin, we need to terminate that shell after requested command -
otherwise the service will not terminate automatically waiting for
further input (next commands).
