My issue is nearly identical to this question. I tried those solution and none worked. But I am using a different Android x86 image. I'm using the ICS (4.0-RC1) asus_laptop image. (I tried a different image previously and couldn't get it to install.)
I installed VirtualBox 4.1.12 on Kubuntu 12.04. I followed these steps and installed the android-x86-4.0-RC1-asus_laptop.iso image in my VM. It boots up and works correctly except for networking.
Alt-F1 netcfg shows no interfaces up except the lo (127.0.0.1). eth0, which should be available, is not shown. That prevents me from trying the solution here.
New references I'm checking out:
The following works very good for me
sudo vi /etc/init.sh
add 4 lines below to init.sh
netcfg eth0 dhcp
echo nameserver <ip> > /etc/resolv.conf
dnsmasq
setprop net.dns1 8.8.8.8
on virtualbox set the network interface to bridged (PCnet Fast III) and that's all.