[Qemu-ppc] Update boot_order on reset for sPAPR

[dvaleev]

This is cleaner attempt to fix -boot once=X behaviour on sPAPR

qemu-system-ppc64 -m 1024 -enable-kvm -boot once=d -vga none -nographic
Ready! 
0 > dev /chosen   ok
0 > .properties  
stdout                           3e512100  
....
ibm,architecture-vec-5           00000000  0080
qemu,boot-device                 d
                                 6400
linux,stdout-path                /vdevice/address@hidden
....
0 > reset-all
....
Ready! 
0 > dev /chosen   ok
0 > .properties  
stdout                           3e512100  
....
ibm,architecture-vec-5           00000000  0080
linux,stdout-path                /vdevice/address@hidden

Compared to previous patch set, this one doesn't implement register_boot_set 
for sPAPR,
but instead updates boot_order at MachineState and read it on each guest's 
reset.