Xenserver master pool broke after UPS meltdown on datacenter.
Other slaves were OK, so elected new pool master via xe pool-emergency-…

Needed to get the VMs running on the master up and running again.

xe vm-list | less
xe vm-reset-powerstate --force uuid=
xe vdi-list | less
xe vdi-forget uuid=
Storage -> Rescan
Reattach VDI on VM
Start VM

Repeat for all machines...

After this procedure, you might need to remove the host from the pool, format it and re-add it again.

VMs running on other hosts may seem OK but suddenly start to randomly loose network connectivity.
To fix this, move the VMs to another host and restart it as soon as you can.