The accrss right of HV RAM can be changed to PAGE_USER (eg. trusty RAM
of post-launched VM). So before using clflush(or clflushopt) to flush
HV RAM cache, must allow explicit supervisor-mode data accesses to
user-mode pages. Otherwise, it may trigger page fault.
Tracked-On: #6020
Signed-off-by: Tao Yuhong <yuhong.tao@intel.com>