Lets also test basic communication primitives. `mock` python module
makes it _much_ easier, so lets use it also on python2 - requiring
backported version installed.
It doesn't make sense to send mgmt call to _named_ VM just to ask for
its name. Use value that QubesVM object already have.
This also means we can safely access vm.name, no need to touch any
private attribute.
QubesOS/qubes-issues#853
