Supersedes #136.

This PR adds:

• Networking UIO driver to libvmm to allow creation of Ethernet driver VMs (maybe WiFi/Cellular in the future?)
• A patched echo server example that uses an Ethernet driver VM.

The basic idea is:

• The sDDF network control, data regions and notification from virtualisers are mapped into Linux userspace via UIO.
• A promiscuous socket is opened to sniff all incoming frames. When a frame comes through from the socket, we enqueue the data into the appropriate sDDF queues and fault on a pre-determined address so the VMM can notify the RX virtualiser.
• When a client wants to transmit data, we will get TX virt notifications via UIO, then we simply write the data out into the socket.

There is a design issue to be aware of: the driver VM needs to know how many network clients there are and map their TX data regions into Linux userspace due to lack of DMA.

TCP performance:
Requested_Throughput,Receive_Throughput,Send_Throughput,Packet_Size,Minimum_RTT,Average_RTT,Maximum_RTT,Stdev_RTT,Median_RTT,Idle_Cycles,Total_Cycles
1000000000,145529249,145506692,1460,56336,63790,71432,1489.88,63770,0,0,0

UDP performance:
Requested_Throughput,Receive_Throughput,Send_Throughput,Packet_Size,Minimum_RTT,Average_RTT,Maximum_RTT,Stdev_RTT,Median_RTT,Idle_Cycles,Total_Cycles
200000000,176977185,200000147,1472,6615,8719,10681,1077.98,9053,0,0,0

Note that this performance depends on an unmerged sDDF network queue library that has compiler barriers.