The old code edited the live XML, called attachDevice and redefined the
domain from it: a stopped VM lost its VNC password, a paused VM crashed
with UnboundLocalError, an unknown CD-ROM was silently ignored (on a
running VM the last disk of the loop could be updated instead), and a
missing ISO raised AttributeError.
Media changes now use updateDeviceFlags on the requested CD-ROM, built
from the live and the persistent definition separately. A CD-ROM
missing from one definition, an unknown ISO, an ISO name present in
more than one pool, or a change that reached only the running VM is
reported to the user. The mount and unmount log messages were swapped.
