Make TYPE_ALPHA_CPU abstract and add types <name>-alpha-cpu.
Use type inheritence, and turn "2*" models into aliases.

Move cpu_alpha_init() to cpu.c and split out CPU realization.
Default to creating type "ev67-alpha-cpu" as before.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Acked-by: NRichard Henderson <rth@twiddle.net>