Since 4dc86310 "Use maxmem=0 to disable qmemman, add more automation to
it" meminfo-writer service is not accessible directly. maxmem property
is used to encode memory management instead.
Allow removing VMs based on multiple prefixes at once. Removing them
separately doesn't handle all the dependencies (default_netvm, netvm)
correctly. This is needed for backup compatibility tests, where VMs are
created with `test-` prefix and `disp-tests-`. Additionally backup code
will create `disp-no-netvm`, which also may need to be removed.
Changed the inheritance hierarchy:
1. Renamed `SystemTestsMixin` to `SystemTestCase`
2. `SystemTestCase` is a child of `QubesTestCase`
3. All classes extending the prior `SystemTestsMixin` now just extend `object`
