vm: add DispVMWrapper for calling a single service in new DispVM
vm: add DispVMWrapper for calling a single service in new DispVM

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

marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client