CA-119099: Don't rely on existence of mount point for SR auto-attach

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Signed-off-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>

(cherry picked from commit b643275944d5912305bd9ff2e3f1ee5a0d16b803)

GitHub: closes #126 on xapi-project/sm