spicevmc: set state of DeviceInstance to NULL
spicevmc: set state of DeviceInstance to NULL

After spice_char_device_state_destroy is called spicevmc should not keep
reference to that memory. state->chardev_st and sin->st point to the
same SpiceCharDeviceState and both should be set to NULL when it is
destroyed.

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