e1000: Choose which set of props to migrate

e1000: Choose which set of props to migrate

When we're using the subsection we migrate both
the 'props' and 'tso_props' data; when we're not using
the subsection (to migrate to 2.11 or old machine types) we've
got to choose what to migrate in the main structure.

If we're using the subsection migrate 'props' in the main structure.
If we're not using the subsection then migrate the last one
that changed, which gives behaviour similar to the old behaviour.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>