CA-137990: allow ring disconnection with pending requests

This patch series allows the ring to be disconnected even if there are pending requests. This allows the front-end to re-open the ring at will without having to take care of cleaning up its pending requests. This is especially important for the Windows front-end that receives the reset signal quite often when I/O is slow.  Although this behaviour seems to be the front-end's responsibility, we choose to fix this at the back-end, as it is far simpler to do so.

After a ring with pending requests has been disconnected, when these requests complete at the back-end with an error that will be ignored and such requests won't be retried.
tapdisk_xenio_ctx_find_blkif is called from tapdisk_xenblkif_find (td_blkif.c, line 44). In the call, _cond has !blkif->dead as one of the conditions.