SVG: Handle dynamically (un)registered network interfaces (2)

SVG: Handle dynamically (un)registered network interfaces (2)

Commit f5d9bce dealt with network interfaces that can be dynamically
registered or unregistered in /proc/net/dev. But it assumed that network
interfaces appear always at the same position in the /proc/net/dev file.
As this is not necessarily the case, this patch makes sure that we
update the right graph corresponding to the same network interface.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>