Docs for vm/task state transitions.

Added diagrams to RST docs that show  required vm_state / task_state
transitions for compute api operations.

(Set Sandy Walsh as author since he created the diagrams)

Change-Id: Ie6a9cd4b54321a98fea5429ee52eb781f57f31a4

doc/source/devref/index.rst

@@ -38,6 +38,7 @@ Background Concepts for Nova
 
    architecture
    threading
+   vmstates
    il8n
    filter_scheduler
    multinic

doc/source/devref/vmstates.rst

@@ -0,0 +1,24 @@
+Virtual Machine States and Transitions
+=======================================
+
+Preconditions for commands
+--------------------------
+
+
+The following diagrams show the required virtual machine (VM) states and
+task states for various commands issued by the user:
+
+
+.. image:: /images/PowerStates1.png
+
+.. image:: /images/PowerStates2.png
+
+
+Create instance states
+----------------------
+
+The following diagram shows the sequence of VM states, task states, and
+power states when a new VM instance is created.
+
+
+.. image:: /images/run_instance_walkthrough.png