m68k: virt: Switch to new sys-off handler API

On m68k with CONFIG_VIRT=y (e.g. virt_defconfig or allmodconfig):

    arch/m68k/virt/config.c: In function ‘config_virt’:
    arch/m68k/virt/config.c:129:2: error: ‘mach_power_off’ undeclared (first use in this function); did you mean ‘pm_power_off’?
      129 |  mach_power_off = virt_halt;
	  |  ^~~~~~~~~~~~~~
	  |  pm_power_off

Commit 05d51e42 ("m68k: Introduce a virtual m68k machine")
introduced a new user of mach_power_off.
Convert it to the new sys-off handler API, too.

Reported-by:  <noreply@ellerman.id.au>
Fixes: f0f7e526 ("m68k: Switch to new sys-off handler API")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>