Move it to qom/cpu.c to avoid build failures depending on include order
of cpu-qom.h and exec/cpu-all.h.

Change opaques of various ..._irq_handler() functions to the
appropriate CPU type to facilitate using cpu_reset_interrupt().

Fix Coding Style issues while at it (missing braces, indentation).
Signed-off-by: NAndreas Färber <afaerber@suse.de>