Move vm.appvms to QubesVM
Move vm.appvms to QubesVM

First, TemplateVM is not used anymore (see previous commit). Second,
don't harcode on client side that "only TemplateVM can be a template for
any VM" (which actually isn't true: AppVM can be a template for DispVM).

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