KVM: x86: fix KVM_SET_XCRS loop

The loop was always using 0 as the index.  This means that
any rubbish after the first element of the array went undetected.
It seems reasonable to assume that no KVM userspace did that.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>