qdev: Track runtime machine modifications

Create a trivial interface to track whether the machine has been
modified since boot.  Adding or removing devices will trigger this
to return true.  An example usage scenario for such an interface is
the rtl8139 driver which includes a cpu_register_io_memory() value
in it's migration stream.  For the majority of migrations, where
no hotplug has occured in the machine, this works correctly.  Once
the machine is modified, we can use this interface to detect that
and include a subsection for the device to prevent migrations to
rtl8139 versions with this bug.

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>
