fix(backups): disable CBT when CBT is unusable

[fbeauchamp]

Description

Short explanation of this PR (feel free to re-use commit message)

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[rtjdamen]

Hi @fbeauchamp, i want to comment on this one. It’s not needed to disable cbt on the vdi itself, just removing all cbt snapshots and run a new full is enough to resolve the problems with invalid cbt here. We have seen some rare issues with disabling CBT on a running VM causing the vdi to get stucked into paused state. So i think it’s better to not run that command unsupervised without knowing what can happen.

[fbeauchamp]

Hi @fbeauchamp, i want to comment on this one. It’s not needed to disable cbt on the vdi itself, just removing all cbt snapshots and run a new full is enough to resolve the problems with invalid cbt here. We have seen some rare issues with disabling CBT on a running VM causing the vdi to get stucked into paused state. So i think it’s better to not run that command unsupervised without knowing what can happen.

this won't be run automatically, at least for now. It will be used when manually disabling Changed Block Tracking , as an additional way to clean up any remaining data, broken cbt chain.
Also calling disable_cbt on the xapi is the only way to make it remove the *.cbtlog file without accessing the storage repository

[b-Nollet]

Looks good to me apart from the little typos in the comments

[b-Nollet]

Suggested change

	// CBt is not usable: reset it
	// CBT is not usable: reset it

[b-Nollet]

Suggested change

	* xapi can't disable CBT on a snapshot OPERATION_NOT_ALLOWED(VDI is a snapshot )
	* xapi can't disable CBT on a snapshot OPERATION_NOT_ALLOWED(VDI is a snapshot)