x86, fpu: Use static_cpu_has() to implement use_xsave()

use_xsave() is now just a special case of static_cpu_has(), so use
static_cpu_has().

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <1273135546-29690-2-git-send-email-avi@redhat.com>