The xenserver-pv-tools RPM that contains the guest tools ISO has a post-installation script called unmount_halted_xstools.sh. As the name says, it unmounts the guest tools ISO only from halted guests. That script is executed when the xenserver-pv-tools is updated.
If the RPM is updated while the ISO is attached to running VMs, those running VMs end-up with a missing VDI and thus:
- live migration fails and has been reported to sometimes crash the VM (migration stuck, forced reboot of the guest necessary to recover)
- stopping then starting the VM gives a missing VDI error
Suggested solution: unmount the guest tools ISO from all VMs, running or not, when the RPM is updated. Unless that could cause serious damage to the running VMs (I don't foresee that but I can be wrong).