spapr: Don't hijack current_machine->boot_order

spapr: Don't hijack current_machine->boot_order

QEMU 6.0 moved all the -boot variables to the machine. Especially, the
removal of the boot_order static changed the handling of '-boot once'
from:

    if (boot_once) {
        qemu_boot_set(boot_once, &error_fatal);
        qemu_register_reset(restore_boot_order, g_strdup(boot_order));
    }

to

    if (current_machine->boot_once) {
        qemu_boot_set(current_machine->boot_once, &error_fatal);
        qemu_register_reset(restore_boot_order,
                            g_strdup(current_machine->boot_order));
    }

This means that we now register as subsequent boot order a copy
of current_machine->boot_once that was just set with the previous
call to qemu_boot_set(), i.e. we never transition away from the
once boot order.

It is certainly fragile^Wwrong for the spapr code to hijack a
field of the base machine type object like that. The boot order
rework simply turned this software boundary violation into an
actual bug.

Have the spapr code to handle that with its own field in
SpaprMachineState. Also kfree() the initial boot device
string when "once" was used.

Fixes: 4b7acd2ac821 ("vl: clean up -boot variables")
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1960119
Cc: pbonzini@redhat.com
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210521160735.1901914-1-groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>