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: I6f82f7cdb0de1f52e64fb80a7a8cb22e5aa23761 Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>