Some useful links for SQL database mirroring with Citrix XenApp are as follows:
Using SQL Database Mirroring to Improve Citrix XenApp Server Farm Disaster Recovery Capabilities: http://support.citrix.com/article/CTX111311
Changing a Production XenApp 7.x Site to use SQL Mirroring: http://carlwebster.com/changing-production-xendesktop-7x-site-use-sql-mirroring/
Troubleshoot Database Mirroring Configuration (SQL Server): https://msdn.microsoft.com/en-IN/library/ms189127.aspx