CA-116545: SR detach could fail during a Find Open process check

This could happen if one pid, out of a list of pids, cease to exist.
Ignoring the error helps us move forward through the list checking
for any pid / file handles

    Signed-off-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>
    Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>
    GitHub: closes #39 on xapi-project/sm