CA-89974: Live migrate fails on IPv6-preferring hosts

Make HTTP client embed IPv6 literal addresses correctly in URLs. A
related commit to xen-api-libs makes the HTTP server understand such URLs.

This commit removes 'Helpers.validate_ip_address' from
xapi/helpers.ml. The related commit to xen-api-libs adds it to
stdext/unixext.ml, renaming it to 'domain_of_addr'.