dma-helpers: Fix too long qiov

If the size of the scatter/gather list isn't a multiple of 512, the
number of sectors for the block layer request is rounded down, resulting
in a qiov that doesn't match the request length. Truncate the qiov to the
new length of the request.

This fixes the IDE qtest case /x86_64/ide/bmdma/short_prdt.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

util/iov.c

@@ -550,3 +550,16 @@ size_t iov_discard_back(struct iovec *iov, unsigned int *iov_cnt,
 
     return total;
 }
+
+void qemu_iovec_discard_back(QEMUIOVector *qiov, size_t bytes)
+{
+    size_t total;
+    unsigned int niov = qiov->niov;
+
+    assert(qiov->size >= bytes);
+    total = iov_discard_back(qiov->iov, &niov, bytes);
+    assert(total == bytes);
+
+    qiov->niov = niov;
+    qiov->size -= bytes;
+}