MFC r302504, r302666, r302668, r302932, r302933:
Add emulation for Intel e1000 (e82545) network adapter.

The code was successfully tested with FreeBSD, Linux, Solaris and Windows
guests. This interface is predictably slower (about 2x) then virtio-net,
but it is very helpful for guests not supporting virtio-net by default.

Thanks to Jeremiah Lott and Peter Grehan for doing original heavy lifting.