CA-125137: sr_ref doesn't always exist

E.g. when doing SR.probe, sr_ref doesn't exist.

Signed-off-by: Zheng Li <zheng.li@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>
CA-125122: Ensure the existance of remotepath when calling _isbind

Signed-off-by: Zheng Li <zheng.li@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>
Merge CP-6697 fixup

Signed-off-by: Zheng Li <zheng.li@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #63 on xapi-project/sm