I've had very good luck running vm's in the unRAID xen system so far. I had not tried Ubuntu 14.04 yet but I have installed Debian and CentOS and didn't have any issues. So I just now installed Ubuntu 14.04 server, updated it and played with it for a few hours. It seems rock solid to me--no networking issues on my install.
My basic .cfg for Linux has a few differences from yours--here is mine, pre-installation:
name = 'ubuntu1404'
builder = 'hvm'
memory = 2048
vcpus = 1
disk = ['qcow2:/mnt/cache/VMs/ubuntu1404/ubuntu1404.qcow2,hda,w' , 'file:/mnt/cache/VMs/iso_store/ubuntu-14.04-server-amd64.iso,hdc:cdrom,r']
boot = 'dc'
vif = ['mac=00:16:3e:00:00:05,bridge=br0']
vnc = 1
vnclisten = '192.168.254.198'
stdvga = '1'
on_poweroff = 'destroy'
on_reboot = 'restart'
on_crash = 'restart'
This is the config which I installed from; I comment out the cdrom iso mount after installation is complete. I think you have some items in your config which are probably only useful to run a Windows OS:
acpi = '1'
apic = '1'
viridian = '1'
xen_platform_pci='1'
sdl = '0'
stdvga = '0'
usb = '1'
usbdevice = 'tablet'
I personally include these settings for Windows VM's but not for Linux. I have two Windows server 2008R2 VM's, Asterisk (CentOS), Debian and now Ubuntu installed in my home unRAID xen system and they all run very predictably, flawlessly and fast.
I'm not sure about the vif setting you use of "model=e1000"---I've never used any setting there and I'm not sure what the outcome intended is. Unless you're trying some kind of hardware pass-through or something like that, you might try putting nothing there after "bridge=br0".
I have to say that I'm very pleased and impressed with how the xen integration is working out with unRAID. I've very grateful to those who have worked to make this happen. It fits a very real need that I was wishing for and it wasn't all that hard to figure out. It would be nice at some point to have a definitive guide put together here for how to install all the major OS's in the unRAID xen host. I had to look in several places to get all the pieces of info I needed.