acpi/ipmi: Initialize the fwinfo before fetching it

The initialization was missed before, resulting in some
bad data in the smbus case.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>