acpi: initialize s4_val used in s4 shutdown

While investigating why a 32 bit Windows 2003 guest wasn't able to
successfully perform a shutdown /h, it was discovered that commit
afafe4bbe0cf7d3318e1ac7b40925561f86a6bd4 inadvertently dropped the
initialization of the s4_val used to handle s4 shutdown.
Initialize the value as before.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Message-id: 1364928100-487-1-git-send-email-brogers@suse.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
