Change in vdsm[master]: vm: Improve error handling when Vm._dom is None

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Change subject: vm: Improve error handling when Vm._dom is None
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Patch Set 3:

* Update tracker::IGNORE, no Bug-Url found
* Check Bug-Url::WARN, no bug url found, make sure header matches 'Bug-Url: ' and is a valid url.
* Check merged to previous::IGNORE, Not in stable branch (['ovirt-3.5', 'ovirt-3.4', 'ovirt-3.3'])