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.
