vhost build fix for i386

vhost.c uses __sync_fetch_and_and(), which is only
available for -march=i486 and above (see
https://bugzilla.redhat.com/show_bug.cgi?id=624279).

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
