x86, fpu: Use the proper asm constraint in use_xsave()

The proper constraint for a receiving 8-bit variable is "=qm", not
"=g" which equals "=rim"; even though the "i" will never match, bugs
can and do happen due to the difference between "q" and "r".

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>