blkdebug: Use QLIST_FOREACH_SAFE to resume IO

Qemu-iotest 030 was broken.

When the coroutine runs and finishes, it will remove itself from the req
list, so let's use safe version of foreach to avoid use after free.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
