KVM: VMX: Add AX to list of registers clobbered by guest switch

By chance this caused no harm so far. We overwrite AX during switch
to/from guest context, so we must declare this.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>