Use qemu/KVM virtualization instead of UML

Guest and network configuration is setup using the libvirt
virtualization API. The [start|stop]_testing scripts have been updated
accordingly.

qemu/KVM does not currently support a hostfs, so the shared build tree
mount has been dropped for now.

testing/config/kvm/vnet1.xml

@@ -0,0 +1,11 @@
+<network>
+  <name>vnet1</name>
+  <uuid>1d6ac7c7-60d9-56c1-a7df-210d3d0cc6d1</uuid>
+  <forward dev='lo' mode='route'>
+    <interface dev='lo'/>
+  </forward>
+  <bridge name='virbr1' stp='on' delay='0' />
+  <mac address='52:54:00:97:F9:FD'/>
+  <ip address='192.168.0.254' netmask='255.255.255.0'>
+  </ip>
+</network>