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.
