Boot modes provide a way for VMs to advertise that they can have their
behavior changed by booting them with a predefined combination of
additional kernel parameters set. This allows qubes to offer features
like a secure system maintenance mode in a way that works seemlessly
with Qubes OS.

This commit documents the features and properties used in boot mode
support, and fixes some automated tests.

When a MockQube is using a tag that is contained in the list of
POSSIBLE_TAGS, its value will always be 0. This fix set the
default 0 value for all possible tags only if it is not explicilty
declared in the list of MockQube tags.

- fix tag application
- fix listing currently attached devices
- fix update-proxy default features

tests should verify whether the desired action has actually been performed

* origin/pr/344:
  q-dev: fix tests/devices
  q-dev: add docstrings and cleanup
  q-dev: denied devices api tests
  q-dev: denied devices api draft
  q-dev: update device_protocol.py

Since some users want pigz, zstd and some other compressions which might
not be installed by default, enable them if they are installed.

Related: QubesOS/qubes-issues#8291
Related: QubesOS/qubes-issues#8211

Wrap `functools.partial` inside `staticmethod` to reduce test report
clutter. Fixing a small typo

* origin/pr/346:
  Cleanup Backup Compatibility test output
  Enable more common compression filters for restoring