CA-150404: iSCSI login fails with non-ASCII CHAP credentials

The fix addresses the problem of not being able to attach an iSCSI based SR to
XenServer, if the target has specified non-ASCII characters for its CHAP
username and/or password.

Usernames: 江月, üöäÜÖÄáÀ
Passwords: 江月江月江月江月江月江月, üöäÜÖÄadmin!23

After the fix:
    * Attaching an iSCSI SR using a WS2012R2 target.
      Can use any non-ASCII characters for username but only ASCII for
      passwords.

    * Attaching an iSCSI SR using an IET target.
      Can use any non-ASCII characters for username and passwords.

Signed-off-by: Kostas Ladopoulos <konstantinos.ladopoulos@citrix.com>
CA-150404: [UnitTest] CHAP credentials are always encoded in UTF-8

Signed-off-by: Kostas Ladopoulos <konstantinos.ladopoulos@citrix.com>