In case you run into the same problem as me (that is, trying to unplug an already unavailable ISO SR from XenServer 5.6 and finding out that the
xenserver-linuxfixup-disk.iso can't be removed), there's a way out. Granted, it's not that easy, but at least it worked for me.
Assumptions taken here: you know how to navigate
xe (the console tools of the XenServer host), knows the
UUID of the SR (using
xe sr-list type=iso) and knows this is all funky stuff that, if done wrong, WILL BREAK things. You've been warned.
Firstly, find out the UUID of the VDI (trying to unplug the PBD or the SR through console should give you that):
xe vdi-list name-label=xenserver-linuxfixup-disk.iso
Find the UUID of the VBD (should return only one):
xe vdi-param-get uuid=<VDI_UUID> param-name=vbd-uuids
Find the UUID VM which is using it:
xe vbd-param-get uuid=<VBD_UUID> param-name=vm-uuid
Shutdown the VM (it takes a while, don't panic and go get a coffee!):
xe vm-shutdown uuid=<VM_UUID>
After that, XenCenter should allow you to Forget/Detach/Destroy the ISO SR, or you can do it through xe:
xe pbd-unplug uuid=<PBD_UUID> xe sr-forget uuid=<SR_UUID>