In spite of vhm_req status could be updated in HV and DM on different CPUs, they
only change vhm_req status when they detect vhm_req status has been updated by
each other. So vhm_req status will not been misconfigured. However, before HV
sets vhm_req status to REQ_STATE_PENDING, vhm_req buffer filling should be visible
to DM. Add a write memory barrier to guarantee this.
Tracked-On: #1842
Signed-off-by: Li, Fei1 <fei1.li@intel.com>