target-ppc: Register all types for TARGET_PPCEMB

Don't attempt to suppress registration of CPU types, since the criteria
is actually a property of the class and should thus become a field.
Since we can't check a field set in a class_init function before
registering the type that leads to execution of that function, guard the
-cpu class lookup instead and suppress exposing these classes in -cpu ?
and in QMP.

In case someone tries to hot-add an incompatible CPU via device_add,
error out in realize.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
