object: release all props

object: release all props

Class properties may have to release resources when the object is
destroyed. Let's use the existing release() callback for that, but
class properties must not release ObjectProperty, as it can be shared
by various instances.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200110153039.1379601-18-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>