If you have problems, that you master is dead or have som troubles, and you need to selected a new master, you can do this with this commands:
xe pool-emergency-transition-to-master
Then on the others slave you can run this commands:
xe pool-emergency-reset-master master-address=NewMasterAddress
And then when the master is running again run this command on the old master
xe pool-emergency-reset-master master-address=NewMasterAddress