cso support

[sundance]

The ability to print and read traces would help a lot for debugging.
So far, I'm limited to code reviewing, which can catch a few "obvious" things, but is insufficient for serious debugging.

Anyway, having fixed a nb of minor trivial bugs in iso_loader code path, probably without visible consequences,
I've now reached a part where I suspect the *.cso related crash may lie.

Selecting a *.cso file (or any other type of image) on GUI
triggers `fs_iso_mount()`,
which, after successful loading and mounting, invokes `virt_iso_init_percd()`.
That's where it seems to go wrong for *.cso.
`virt_iso_init_percd()` invokes `get_toc_and_lba()`,
of which 2 versions exist in source code, but one is commented, hence only one is valid.
There is no explanation as to why the other version is commented out though left in the code.

Comparing these 2 versions, the code paths for *.cso look very different.

The new code invokes `get_lba_from_mki()` to determine `lba`.
If it doesn't, it unconditionally set it to 150 (a magic number without a name),
and there is this an associated comment :
// TODO: isofile_find_lba(ifs);

which looks suspicious.

This only matters if `get_lba_from_mki()` fails, but without traces, it's difficult to determine if it's what happens.