Very few calls at client side really needs VM class name. So, even in
non-blind mode use just QubesVM class, to avoid strange cases depending
on blind mode being enabled or not. Then, have VM class name in 'klass'
property. If known at object creation time, cache it, otherwise query
qubesd at first access.
Abort tar process after extracting requested files - do not parse the
archive until the end (possibly tens of GB later).
FixesQubesOS/qubes-issues#2986
The most important part is fixing resize handling - call size_func
before data_func, but after tar gets initial data (and output file
size).
But other than that, it makes the process a little faster.
QubesOS/qubes-issues#1214
Opt for a simple one-liner error messages, instead of meaningless stack
trace (it's most of the time about qubesd responding with error, so the
stack trace of actual problem is elsewhere).
