cpu: Turn cpu_unassigned_access() into a CPUState hook

Work around *-user's lack of hwaddr.

Signed-off-by: Andreas Färber <afaerber@suse.de>
cpu: Replace cpu_single_env with CPUState cpu_single_cpu

Fiddle with include order to ensure CPUState is defined before
cpu_single_cpu.

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