Use correct bool properties parsing by default
Boolean properties require specific setter to properly handle literal
"True" and "False" values. Previously it required all bool properties to
include 'setter=qubes.property.bool' in addition to 'type=bool'.
This fixes loading some boolean properties from qubes.xml. Specifically
at least include_in_backups on DispVM class lacked setter, which
resulted in property being reset to True automatically on qubesd
restart.
Fixes QubesOS/qubes-issues#4831