[B2-devel] support for libvhd

[Geoffrey B. <geo...@in...>]

For my work on digital preservation it's important to have "golden"
disk images that are not
corrupted by user action.  In order to enable this, I've added support
for VHD virtual disks (especially snapshots !)
to the Linux and OS X versions of BasiliskII and SheepShaver.  The support uses
the open source libvhd library which is part of xen (available here
(http://www.xen.org/products/xen_source.html))

The piece that's needed is libvhd which is in tools/blktap2 and it can
be separately compiled.
The vhd-util enables creation of vhd disks and snapshots.

Compiling libvhd for OS X is non-trivial and required  1) a new config
and 2) a number of small changes
to the include files and c files.  Compiling for linux is a snap.

I use this as follows.

1) create my "golden image"  gold.dsk in the usual way
2) create a snapshot:  vhd-util snapshot -n gold.vhd -p gold.dsk -m
3) use the snapshot in my prefs file

In my work the golden images are in an AFS system which means the golden images
can reside at "universal" addresses.   The snapshots are initially
tiny, so a complete
virtual machine configuration -- prefs + snapshot is quick to download
for the end user.
The snapshots are copy on write which has the pleasant side effect of
letting the end user
keep any changes.

I've attached my diffs and files.  One change to
SheepShaver/src/Unix/main_unix.cpp is there just
to work around a bug in the Linux implementation -- on my system mmap
with MAP_FIXED causes
a seg fault.

Geoffrey