Fix a multi HVM domain issue (bugzilla bug #542):

Fix a multi HVM domain issue (bugzilla bug #542):
        This issue can be reproduced on SMP platform, while the domain 0
is UP. The reason is, after finishing a dma request, the dma thread will
trigger the interrupt and then clear the call back function. When guest
get the interrupt , it will try to check the call back function, if it
is set, then it will trigger a dma request again. So if the checking for
the callbackfunction happens between trigger interrupt and clear
callback function on dma thread, it will cause NULL function calling.
After with domain 0 UP and smp platform, this situation can be
reproduced easily.

Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>