[Libvir] default hypervisor selection

[Richard W.M. Jones]

I'm mostly with Dan Berrange here.  Surely an environment variable is
the right thing to do, and then later we can add some advanced probing
if the environment variable alone doesn't satisfy users.

The only problem I see is making the NULL case unpredictable or
introducing unreproducible bugs.  But I guess that's not very likely.

Dan's patch
(http://www.redhat.com/archives/libvir-list/2008-January/msg00234.html)
is fine except it should include a DEBUG() statement so we have some
chance to gather information from bug reports about what it was
actually trying to connect to.  At the moment, the first DEBUG() isn't
done until after the URI is parsed, and URI parsing is quite likely to
be a failure point.

Rich.

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