Changes to speed up qvm-ls

[qubesuser]

This is the client part of a bunch of changes that get qvm-ls to around 200ms run time.

Warning: I haven't tested them much, and they change fundamental code, so they probably need fixes.

In particular, all properties are now cached by default, so anything that expects to see changes is now broken and needs to be fixed.

This makes use of the new APIs introduced in QubesOS/qubes-core-admin#165, and also contains a bunch of optimizations of the Python code.

[marmarek]

I've cherry-picked two more commits from this:

• don't lookup list of labels just to read VM properties 6eb828f
• don't use ast.literal_eval, just directly convert to the desired type 10c4c2f