Fix race where a bm can get kicked off twice, resulting in an ioemu crash
because the bm is no longer valid.

diff -r a2b4ffd594a8 tools/ioemu/hw/ide.c
--- a/tools/ioemu/hw/ide.c	Thu Jun 14 14:26:39 2007 +0100
+++ b/tools/ioemu/hw/ide.c	Thu Jun 14 14:30:51 2007 +0100
@@ -427,7 +427,8 @@ static void *dma_thread_func(void* opaqu
         if (rv != 0) {
             if (read(file_pipes[0], &req, sizeof(req)) == 0)
                 return NULL;
-            dma_thread_loop(req);
+	    if (req->status & BM_STATUS_DMAING)
+		dma_thread_loop(req);
         } else {
             if (suspend_requested)  {
                 /* Need to tidy up the DMA thread so that we don't end up