char device: use _reset_dev_instance() to set 'sin'
char device: use _reset_dev_instance() to set 'sin'

Internally, use the method to set the 'sin' member variable so that we
don't have to duplicate the g_object_notify() calls, and there are
consistent debug statements whenever this value is modified. This also
means that we need to handle NULL arguments to this function.

Acked-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>

SPICE/spice,fgouget/spice,fgouget/spice,SPICE/spice,fgouget/spice,fgouget/spice,SPICE/spice