The ne2000-isa device defines a VMState struct for migration, but
we forgot to actually register it. Correct this deficiency by
setting dc->vmsd.
Change-Id: If637bfc5923df0ae28a89fde16475a87f57b320b Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>