make_request sets the request bit, and signal_event wakes the vcpu
thread. If we signal_event comes first, the target vCPU has a chance to
sleep again before processing the request bit.
Tracked-On: #8507
Signed-off-by: Wu Zhou <wu.zhou@intel.com>
Reviewed-by: Junjie Mao <junjie.mao@intel.com>