Disk extend during migration

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as a follow-up of BZ #1883399 [1], we are reviewing vdsm VM migration flows and 
solve few follow-up bugs, e.g. BZ #1981079 [2]. I have couple of questions 
related to libvirt:

* if we run disk extend during migration, it can happen that migration finishes 
sooner than disk extend. In such case we will try to set disk threshold on 
already stopped VM (we handle libvirt event that VM was stopper, but due to 
Python GIL there can be a delay between obtaining appropriate signal from 
libvirt  and handling it). In such case we get libvirt 
VIR_ERR_OPERATION_INVALID when setting disk threshold. Is it safe to 
catch this exception and ignore it or it's thrown for various reasons and the 
root cause can be something else than stopped VM?

* after disk extend, we resume VM if it's stopped (usually due to running out 
of the disk space). Is it safe to do so also when we do the disk extend during 
migration and VM can be stopped because it was already migrated? I.e. can we 
assume that libvirt will handle such situation and won't resume VM in such 
case? We do some checks before resume and try to avoid situation when we 
resume migrated VM, but there can be some corner cases and it would be useful 
to know if we can rely in libvirt to prevent resuming VM in unwanted cases 
like one when VM is stopper after migration.


[1] https://bugzilla.redhat.com/1883399
[2] https://bugzilla.redhat.com/1981079

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