Fix bug in storage/__init__.py dropping "False" config values

[andersonarc]

The _sanitize_config function drops "False" values from the config, resulting in properties such as rw="False" disappearing. As a consequence, the default value of "True" gets reassigned to them. Similar to #829, but does not have any practical consequences I can think of beyond having the correct/expected behavior for the "False" values.

To reproduce:

[user@dom0 ~]$ sudo systemctl stop qubesd
[user@dom0 ~]$ sudo nano /var/lib/qubes/qubes.xml # Set rw="False" for any domU volume
[user@dom0 ~]$ sudo systemctl start qubesd 
[user@dom0 ~]$ # At this stage, rw="False" is still present
[user@dom0 ~]$ # Now, make any edit to the domU - the simplest example is to change the vCPU count, and then apply it
[user@dom0 ~]$ # Then, rw="False" will fully disappear in qubes.xml
[user@dom0 ~]$ sudo systemctl restart qubesd 
[user@dom0 ~]$ # Now, make another edit to the domU to trigger a fresh save
[user@dom0 ~]$ # Now rw="True" appears in qubes.xml

Pinging @marmarek for review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.47%. Comparing base (befbb9b) to head (35bf9c3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #834   +/-   ##
=======================================
  Coverage   70.46%   70.47%           
=======================================
  Files          61       61           
  Lines       14161    14162    +1     
=======================================
+ Hits         9979     9980    +1     
  Misses       4182     4182           

Flag	Coverage Δ
unittests	70.47% <100.00%> (+<0.01%)	⬆️

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