kvm/ppc/mpic: fix mmio region lists when multiple guests used

Keeping a linked list of statically defined objects doesn't work
very well when we have multiple guests. :-P

Switch to an array of constant objects.  This fixes a hang when
multiple guests are used.

Signed-off-by: Scott Wood <scottwood@freescale.com>
[agraf: remove struct list_head from mem_reg]
Signed-off-by: Alexander Graf <agraf@suse.de>