When a VM (or its template) does not explicitly set a qrexec_timeout,
fall back to a global default_qrexec_timeout (with default value 60),
instead of hardcoding the fallback value to 60.
This makes it easy to set a higher timeout for the whole system, which
helps users who habitually launch applications from several (not yet
started) VMs at the same time. 60 seconds can be too short for that.
Since we have app.default_pool* properties, create appropriately named
pool and let those properties choose the right pool. This also means we
don't need to specify pool name in default volume config anymore
QubesOS/qubes-issues#2256
Since qvm-prefs is now a generic tool without any property-specific
stuff, we can tests properties getters/setters in QubesVM class unit
tests.
Now most of properties are tested. Some of those tests still fails
because missing/incomplete underlying implementation.
