ipmi: chassis poweroff should use qemu_system_shutdown_request()

ipmi: chassis poweroff should use qemu_system_shutdown_request()

When issuing a chassis 'powerdown' control command, the routine
qemu_system_shutdown_request() should be used to exit the guest.
qemu_system_powerdown_request() will initiate a soft shutdown which is
not what is required by the IPMI (28.3 Chassis Control Command):

    0h = power down. Force system into soft off (S4/S45) state. This
    is for 'emergency' management power down actions. The command does
    not initiate a clean shut-down of the operating system prior to
    powering down the system

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>