resources: virt: Add a virt resource based on libvirt

This adds an initial implementation of a virt resource based on libvirt.
It is not complete and requires more testing. The initial skeleton was
written by nseps but was not merged. It was later cleaned up and merged
in its current form by purpleidea. Many thanks to nseps for getting this
going, and hopefully we'll get you contributing more in the future!

examples/virt1.yaml

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+---
+graph: mygraph
+resources:
+  virt:
+  - name: mgmt1
+    uri: 'qemu:///session'
+    cpus: 1
+    memory: 524288
+    state: shutoff
+    transient: true
+edges: []

examples/virt2.yaml

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+---
+graph: mygraph
+resources:
+  virt:
+  - name: mgmt2
+    uri: 'qemu:///session'
+    cpus: 1
+    memory: 524288
+    state: shutoff
+    transient: false
+edges: []

examples/virt3.yaml

@@ -0,0 +1,11 @@
+---
+graph: mygraph
+resources:
+  virt:
+  - name: mgmt3
+    uri: 'qemu:///session'
+    cpus: 1
+    memory: 524288
+    state: running
+    transient: false
+edges: []