hw/net/ne2000: extract ne2k-isa code from i386/pc to ne2000-isa.c

- add "hw/net/ne2000-isa.h"
- remove the old i386 dependency

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au> [PPC]
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>