Report the default value associated with a property.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200110153039.1379601-26-marcandre.lureau@redhat.com>
[Report it as type "any", not string. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>