There's now a couple of options for specifying what type of hardware you want attached to your virtual machine.
Some guest OSes may lack drivers for certain network cards. KVM/QEMU has a few different types available, you can choose which one you want to use with the "NIC device" dropdown on the VM management screen.
To save changes, make sure you click the red button to the right of the dropdown (I've made the mistake a couple of times of changing the option in the dropdown and just hitting power on). Also, note that if your VM is running, the change will not be instantaneous. You'll need to power off and power back on your VM for the change to take effect (think about it - would you change the ethernet card in a real machine while it was switched on?).
If you are getting timeout or dropped connections when transferring large files or anything else related to network issues try setting virtio as your NIC driver before escalating it as a bug (?)
To be honest, I'm not entirely sure what this does. Ticking the box makes RBVM set the -no-kvm-irqchip command-line option when launching your VM. The internet told me that this fixes lots of obscure problems that people have with some of the less common guest OSes (like OpenBSD), so it seemed like a good idea to include a way to set it.
Like the NIC device selection, you need to click the red button to the right of the checkbox after changing it (again, I've made the mistake of ticking it and just hitting power on, and then wondered for ten minutes why it didn't work). Also, the change only takes effect when you power on your VM - if it's running, you need to power it off and then on again.