Issue starting a VM on Citrix Xen after an outage

After a recent problem with our Citrix Xen cluster we had one Windows VM that refused to start up:

[root@XENSERVER1 ~]# xe vm-start uuid=4dc9528c-0eb7-38fc-7246-66b387d6aa0e
Error code: SR_BACKEND_FAILURE_46
Error parameters: , The VDI is not available [opterr=VDI aae84c26-4520-45cd-ad66-5e379874f5dd already attached RW],

After a lot of poking and prodding the VM's disks seemed fine.
I tried to export the VM and received this message:

[root@XENSERVER1 ~]# xe vm-export uuid=4dc9528c-0eb7-38fc-7246-66b387d6aa0e filename=/var/run/sr-mount/3184f86f-9743-b819-5cd5-e84ccf7e7c6c/win-server-export.vdi
The server failed to handle your request, due to an internal error. The given message may give details useful for debugging the problem.
message: Failure("The VDI aae84c26-4520-45cd-ad66-5e379874f5dd is already attached in RW mode; it can't be attached in RO mode!")

That got me on the right track and I did this:

[root@XENSERVER1 ~]# xe-toolstack-restart
Stopping xapi: .. [ OK ]
Stopping the v6 licensing daemon: [ OK ]
Stopping the memory ballooning daemon: [ OK ]
Stopping perfmon: [ OK ]
Stopping the fork/exec daemon: [ OK ]
Stopping the multipath alerting daemon: [ OK ]
Starting the multipath alerting daemon: [ OK ]
Starting the fork/exec daemon: [ OK ]
Starting perfmon: [ OK ]
Starting the memory ballooning daemon: [ OK ]
Starting the v6 licensing daemon: [ OK ]
Starting xapi: ....start-of-day complete. [ OK ]
done.

And presto the VM booted:

[root@XENSERVER1 ~]# !250
xe vm-start uuid=4dc9528c-0eb7-38fc-7246-66b387d6aa0e

The problem was that XENSERVER1 was the only one that was not rebooted during the outage since it was the master.