libxc: stop migration in case of p2m list structural changes

libxc: stop migration in case of p2m list structural changes

With support of the virtual mapped linear p2m list for migration it is
now possible to detect structural changes of the p2m list which before
would either lead to a crashing or otherwise wrong behaving domU.

A guest supporting the linear p2m list will increment the
p2m_generation counter located in the shared info page before and after
each modification of a mapping related to the p2m list. A change of
that counter can be detected by the tools and reacted upon.

As such a change should occur only very rarely once the domU is up the
most simple reaction is to cancel migration in such an event.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>