tmp105: QOM'ify

Introduce TYPE_ constant and cast macro.
Move the state struct to the new header to allow for future embedding.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>