Revert "CP-7904 - Remove v6 from xe-toolstack-restart."

Put v6 back into xe-toolstack-restart now that it's back in the build.

This reverts commit a174a4346d8b907e7e792ce6b87306acf280d770.
Merge pull request #1732 from johnelse/re-add-v6d

Re-add v6d to xe-toolstack-restart
CA-133718: Fix HIMN login_with_password

HIMN had been relying on login_with_password working on the slave
as it was erroneously not being forwarded. When xapi was changed
to disallow logins on slaves, this broke. This changeset causes
login_with_password to be forwarded to the master, which should
fix the issue.

Imported from xen-api.git f3c578f0c8ce724891925c2bd9beb3be66b83e4f

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Imported-by: John Else <john.else@citrix.com>

Conflicts:
	ocaml/xapi/api_server.ml