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