Use the common function introduced earlier, and report default value.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200110153039.1379601-27-marcandre.lureau@redhat.com>
[Sort the properties, following what is done for -object ...,help. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>