The eth_vde interface is already in the cvs of the mainstream bochs distribution.
See bochs.sf.net.

eth.cc.diff is a patch file for eth.cc.
eth_vde.cc is the driver.

The makefile of the iodev directory must be changed as well.

In the bochrc file the configuration lines for vde should sound like this:
ne2k: ioaddr=0x280, irq=9, mac=fe:fd:00:00:00:01, ethmod=vde, ethdev=/tmp/vde.ctl