rtl8139: Use subsection to restrict migration after hotplug

rtl8139: Use subsection to restrict migration after hotplug

rtl8139 includes a cpu_register_io_memory acquired value in it's
migration data.  This is not only unecessary, but we should treat
these values as unique to the VM instances since the value depends
on call order.  In most cases, this miraculously still works.
However, if devices are added or removed from the system, it may
represent an ordering change, which could cause the target rtl8139
device to make use of another device's cpu_register_io_memory value.
If we detect that a hot-add/remove has occured, include a subsection
to restrict migrations only to driver versions known to include this
fix.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>