qom: Use command line syntax for default values in help

qom: Use command line syntax for default values in help

object_property_help() uses the conventional command line syntax instead
of the JSON syntax. In particular,
- Key-value pairs are written in the command line syntax.
- bool description passed to the function says on/off instead of
  true/false.

However, there is one exception: default values are formatted into JSON.
While the command line and JSON syntaxes are consistent in many cases,
there are two types where they disagree:

string: The command line syntax omits quotes while JSON requires them.

bool: JSON only accepts true/false for bool but the command line syntax
      accepts on/off too, and on/off are also more popular than
      true/false. For example, the docs directory has 2045 "on"
      occurances while it has only 194 "true" occurances.
      on/off are also accepted by OnOffAuto so users do not have to
      remember the type is bool or OnOffAuto to use the values.

Omit quotes for strings and use on/off for bools when formatting
default values for better consistency.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Link: https://lore.kernel.org/r/20250207-bool-v1-1-5749d5d6df24@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>