qom: Introduce CPU class

qom: Introduce CPU class

Reintroduce CPUState as QOM object: It's abstract and derived directly
from TYPE_OBJECT for compatibility with the user emulators.
The identifier CPUState avoids conflicts between CPU() and the struct.

Introduce $(qom-twice-y) to build it separately for system and for user
emulators.

Prepare a virtual reset method, (re)introduce cpu_reset() as wrapper.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>