hw/pc_sysfw: Fix memory leak

Valgrind reported this memory leak which occured a few times.

Test scenario:

qemu-system-i386 (no arguments), only BIOS started, terminate with
monitor command (quit).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>
