VT-d/qinval: make local variable used for communication with IOMMU "volatile"

Without that there is - afaict - nothing preventing the compiler from
putting the variable into a register for the duration of the wait loop.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/passthrough/vtd/qinval.c
+++ b/xen/drivers/passthrough/vtd/qinval.c
@@ -196,7 +196,7 @@ static int queue_invalidate_wait(struct 
     u8 iflag, u8 sw, u8 fn)
 {
     s_time_t start_time;
-    u32 poll_slot = QINVAL_STAT_INIT;
+    volatile u32 poll_slot = QINVAL_STAT_INIT;
     int index = -1;
     int ret = -1;
     unsigned long flags;