Found by review (Codex and agy) of the XML editor's dialog:
- A busy or moved VM made the page reload and the edit was lost. The
dialog's requests now get these answers as JSON (X-Requested-With, as the
create page's flavor forms) and show them; any other failure says the
edit is still in the editor.
- libvirt finds the VM to define by the XML's UUID: another VM's UUID
overwrote that VM past its lock, a new or missing one added a VM. The
XML must keep this VM's UUID; a malformed one is refused with the
parser's error, as on the create page.
- Save sends the XML whose diff the dialog shows, also when two previews
answer out of order; a failed save keeps the diff and Save.
- The edited XML is defined with the line breaks libvirt gave, not the
form's CRLF, and the VM's state is read before the definition, so a
failed read cannot turn a saved change into an error.
The XML editor of a running or paused VM only showed its XML. It now
shows the persistent definition, the one the VM starts with next (with
secrets, like the other edit forms), and Change saves it through the same
review dialog; the running VM keeps its live configuration until it
restarts. The tab and the saved message say so. The check against a
definition changed since the editor was opened covers this case too.
Change used to define the edited XML at once. It now opens a dialog with
the lines the edit changes in the VM's definition, and Save there defines
it. A definition libvirt refuses comes back to the dialog with the reason,
and the edit stays in the editor; before, the page reloaded with the
current XML and the edit was lost. A definition that changed since the
editor was opened (another tab, a disk added) is not overwritten: the
dialog asks to reload. An unchanged XML has nothing to save.
change_xml answers the dialog with JSON; the editor's script moves to
static/js/xml_editor.js.
