XenServer: Join A Domain.

xe pool-enable-external-auth auth-type=AD service-name=<domain.com> config:user=<username> config:pass=<password>

XenServer: Add search domain to DNS.

xe pif-list host-name-label=<hostname> management=true

xe pif-param-set uuid=<uuid> other-config:domain=domain.com

reboot

hostname -d -v

Should show the search domains at the bottom.

Alternatively:

cat /etc/resolv.conf