docs: document cpu host-passthrough migration issue
docs: document cpu host-passthrough migration issue

Documents in formatdomain.html that when migrating a guest
defined with the host-passthrough CPU model from a machine that
is running on a newer CPU model than the destination machine's
CPU model, it is very likely that the guest will crash upon
arrival.

Signed-off-by: Jason J. Herne <937084fec1b14b928d3b598c69ed20c9b59ffea8@linux.vnet.ibm.com>

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