tools: make qvm-ls --fields accept property names
tools: make qvm-ls --fields accept property names

Since migration to Admin API, qvm-ls no longer have list of all VM
properties in advance, so can't really validate fields list. Simply
assume that unknown columns are properties.

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