storage: filter out ventoy image when building storage config
When ventoy is used, the filesystem mounted to /cdrom is not a /dev/sr0
device (major number 11) but a dm device named "ventoy". Add a check to
exclude it as well as real cdrom devices.
system-install: use --assume-downloaded instead of --no-download
Running $ apt-get install --no-download
fails if the packages were previously "downloaded" using the cdrom
rather than an online repository.
This means we can't reliably pass the option on the second apt-get
install invocation.
Passing the --no-download option to $ curtin system-install while not
passing it to apt-get feels wrong. Therefore, I've renamed the
system-install option from --no-download to --assume-downloaded.
For package managers that support the --no-download option properly,
they can use it on the second invocation. For apt, we will not pass the
option.
apt-config: stop returning cfg object in translate_old_apt_features
The translate_old_apt_features function mutates its parameter, but also
used to return the translated object. This goes against the rules of
idiomatic Python. Returning a new configuration object without mutating
the original one would require leaning on copy.deepcopy, which is
expensive and not always needed.
Let's stop returning the resulting configuration so the interface is
clear.
