hw/net/ne2000: Extract the PCI device from the chipset common code

The ne2000.c file contains functions common the the ISA and PCI
devices. To allow to build with one or another, extract the PCI
specific part into a new file.

This fix an issue where the NE2000_ISA Kconfig had to pull the
full PCI core objects.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190504123538.14952-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>