setting back is-a-template=false on a VM causes inconsistent state

[ydirson]

Probably this should be refused, maybe that param ought to be set to RO once set?

[11:01 xcpng83-bzkcpvhy xcp]# xe vm-import   url=http://pxe/images/alpine-uefi-minimal-3.12.0.xva sr-uuid=3c1b62e9-af3f-62e1-b9b9-cad7e56505ed 
41346790-5f5f-155a-61a8-1a5d15c227da
[11:01 xcpng83-bzkcpvhy xcp]# xe vm-param-get param-name=is-a-template uuid=41346790-5f5f-155a-61a8-1a5d15c227da 
false
[11:02 xcpng83-bzkcpvhy xcp]# xe vm-param-set is-a-template=true uuid=41346790-5f5f-155a-61a8-1a5d15c227da 
[11:02 xcpng83-bzkcpvhy xcp]# xe vm-param-get param-name=is-a-template uuid=41346790-5f5f-155a-61a8-1a5d15c227da 
true
[11:03 xcpng83-bzkcpvhy xcp]# xe vm-param-set is-a-template=false uuid=41346790-5f5f-155a-61a8-1a5d15c227da 
[11:03 xcpng83-bzkcpvhy xcp]# xe vm-param-get param-name=is-a-template uuid=41346790-5f5f-155a-61a8-1a5d15c227da 
false
[11:03 xcpng83-bzkcpvhy xcp]# xe vm-param-set is-a-template=true uuid=41346790-5f5f-155a-61a8-1a5d15c227da 
You gave an invalid object reference. The object may have recently been deleted. The class parameter gives the type of reference given, and the handle parameter echoes the bad value given.
class: VM_metrics
handle: OpaqueRef:NULL