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>

include/qemu-common.h

@@ -329,6 +329,7 @@ size_t qemu_iovec_memset(QEMUIOVector *qiov, size_t offset,
                          int fillc, size_t bytes);
 ssize_t qemu_iovec_compare(QEMUIOVector *a, QEMUIOVector *b);
 void qemu_iovec_clone(QEMUIOVector *dest, const QEMUIOVector *src, void *buf);
+void qemu_iovec_discard_back(QEMUIOVector *qiov, size_t bytes);
 
 bool buffer_is_zero(const void *buf, size_t len);