The arch_vcpu->irq_window_enabled is almost useless in original code.
This patch use it to avoid unnecessary "interrupt-window exiting"
conditions check if "interrupt-window exiting" is aleady enabled.
Tracked-On: #1190
Signed-off-by: Yu Wang <yu1.wang@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>