VT-d: don't disable VT-d engine in suspend for security purpose

force_iommu option is used to force enabling and using IOMMU for
security purpose. So when force_iommu is set, it shouldn't disable
VT-d engines in suspend.

Signed-off-by: Weidong Han <weidong.han@intel.com>

xen/drivers/passthrough/vtd/iommu.c

@@ -1877,6 +1877,10 @@ void iommu_suspend(void)
         iommu_state[i][DMAR_FEUADDR_REG] =
             (u32) dmar_readl(iommu->reg, DMAR_FEUADDR_REG);
 
+        /* don't disable VT-d engine when force_iommu is set. */
+        if ( force_iommu )
+            continue;
+
         iommu_disable_translation(iommu);
 
         if ( iommu_intremap )