SMM: Disable use of SMM when running under Xen

Xen does not support SMM mode.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
diff --git a/src/smm.c b/src/smm.c
index 72e5e88..d0d1476 100644
--- a/src/smm.c
+++ b/src/smm.c
@@ -10,6 +10,7 @@
 #include "config.h" // CONFIG_*
 #include "ioport.h" // outb
 #include "pci_ids.h" // PCI_VENDOR_ID_INTEL
+#include "xen.h" // usingXen
 
 ASM32FLAT(
     ".global smm_relocation_start\n"
@@ -151,6 +152,8 @@
         return;
     if (!CONFIG_USE_SMM)
         return;
+    if (usingXen())
+	return;
 
     dprintf(3, "init smm\n");
     pci_find_init_device(smm_init_tbl, NULL);