vtd: Add checking on return value of init_vtd_hw()

This ensures correct actions will be taken if init_vtd_hw()
fails. Especially, when 'iommu=force' is used, Xen should not
boot if init_vtd_hw() fails.

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

xen/drivers/passthrough/vtd/iommu.c

@@ -1789,7 +1789,8 @@ int intel_vtd_setup(void)
     memset(domid_bitmap, 0, domid_bitmap_size / 8);
     set_bit(0, domid_bitmap);
 
-    init_vtd_hw();
+    if ( init_vtd_hw() )
+        goto error;
 
     register_keyhandler('V', dump_iommu_info, "dump iommu info");