vt82c686: Make vt82c686b-pm an abstract base class and add vt8231-pm based on it

The vt82c686b-pm model can be shared between VT82C686B and VT8231. The
only difference between the two is the device id in what we emulate so
make an abstract via-pm model by renaming appropriately and add types
for vt82c686b-pm and vt8231-pm based on it.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <34969fc7be984fa070479bfb9f748993a0aef31b.1610223397.git.balaton@eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>