hwaddr: Make hwaddr type usable beyond softmmu

While not normally needed for *-user, it can safely be used there since
always based on uint64_t, to avoid ifdeffery.

To avoid accidental uses, move the guards from exec/hwaddr.h to its
inclusion sites.  No need for them in include/hw/.

Prepares for hwaddr use in qom/cpu.h.

Signed-off-by: Andreas Färber <afaerber@suse.de>
cpu: Turn cpu_unassigned_access() into a CPUState hook

Use it for all targets, but be careful not to pass invalid CPUState.
cpu_single_env can be NULL, e.g. on Xen.

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