qubes: allow passing name of class to app.add_new_vm
qubes: allow passing name of class to app.add_new_vm

This will allow more flexible API usage, especially when using mgmt API
- we need to use VM type as string there.
We don't lose any flexibility here - VM class names needs to be uniquely
identified by a string (used in qubes.xml) anyway.

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin