Prior to this commit, XEN guests disks were single partition, no MBR,
the bits dumped into the lvm. This makes a snapshot of a XEN node,
look completely different then a physical disk image, especially if
users want more disk space (mkextrafs) inside the guest, and then want
to take a snapshot of that, and then run it on a physical node (which
was not possible).

With these changes, guests now use the same MBR layout as our version
two MBR, which makes them interchangeable with physical disk images.
In fact, the goal is to be able to switch back and forth as needed,
based on physical resource availability.

they enter and leave projects. We were leaving lots of files in
the guest group.

    <sliver_type name="emulab-xen">
      <emulab:xen cores="1" ram="512" disk="8"/>
    </sliver_type>

We currently ignore cores ... Ram in MB, disk in GB.

We were assuming that if static_widearea_config returned 0, it had
configured the physical $iface that findcnet was passed successfully.
But, static_widearea_config also returns 0 if the widearea config info
was present BUT if the given physical $iface did not match the widearea
config info.

So, catch that case, and all is well.

Also, fix a minor syntax bug; and add some debugging.

Part of our hack strategy to detect and correct transient NFS errors
is to fsync after every file write and check for errors. However, some
devices don't support fsync. So, for example, if you were trying to
imagezip to /dev/null (to benchmark or check the resulting image size)
fsync would always fail and we would keep retrying the write forever.

There is already a "-N" option to avoid such checks and retries, but
Mike can never remember that!

The script still blows up on a glob of an empty directory.

What source to read the info from and which enternal array to fill.

Controlled by new sitevars.

if we have a small device, such as a USB drive, don't pretend that we know what r/w speed, serial or type.

Check the version of bash we are about to run. It must be version 4
to support associative arrays.