CA-128460: Don't pause tapdisk in the toolstack

This only affects the storage migration code path.

We were pausing in order to flush all pending requests before
checking to see if any of them failed to mirror correctly. Now
tapdisk has exposed a 'reqs_outstanding' counter. We loop
waiting for this to go to zero, then check the
'nbd_mirror_failed' flag.

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>