CP-7916: Reinstate XenAPI WLB code back

Signed-off-by: Phus Lu <phus.lu@citrix.com>
Imported-by: John Else <john.else@citrix.com>

(cherry picked from commit c6f9d09fe0f6b91300c26f388cec2f3bdf6cddec)

Conflicts:
	ocaml/xapi/xapi_db_upgrade.ml
	ocaml/xapi/xapi_host.ml
	ocaml/xapi/xapi_vm_helpers.ml
CP-8155: Code change for xapi and v6d in Creedence to make WLB as a licensed feature

Signed-off-by: Phus Lu <phus.lu@citrix.com>
Imported-by: John Else <john.else@citrix.com>

(cherry picked from commit 94703808d4c58c2ef601d39271f160a2f3145507)
CP-8155: Code change for xapi and v6d in Creedence to make WLB as a licensed feature

Signed-off-by: Phus Lu <phus.lu@citrix.com>
Imported-by: John Else <john.else@citrix.com>

(cherry picked from commit 65f660c774a925e2d82af74f0802bab87ffa309d)
CA-135179: xapi shall raise error but not clean database when disconnect WLB failure.

Signed-off-by: Zheng Chai <zheng.chai@citrix.com>
Imported-by: John Else <john.else@citrix.com>

(cherry picked from commit 8fe838e12e8482af9e81bb4ccd8c16b58089efd8)
CA-137044: XAPI shall handle the WLB disconfigure error according to CA-60147,CA-93312

Signed-off-by: Zheng Chai <zheng.chai@citrix.com>
Imported-by: John Else <john.else@citrix.com>

(cherry picked from commit 8105fe781a98964905c74fff85d23eba5e5046c8)