Log Message:
Turn a non-fatal condition from a panic into a warning, pulling up
a 3.4 change
<http://xenbits.xensource.com/hg/staging/xen-3.4-testing.hg/rev/60ed1226a37c#>
as proposed by cegger (see
<http://mail-index.netbsd.org/port-xen/2011/04/01/msg006580.html> ff).