Add shelve and unshelve info into devref doc

The diagram on http://docs.openstack.org/developer/nova/devref/vmstates.html
does not currently show the "SHELVED_OFFLOADED" state and the
associated "shelve", "shelveOffload" and "unshelve" state transitions.

Change-Id: I50eb07a84336160b8120ca294598576eeb982d28
Closes-Bug: #1394215
Merge "Add shelve and unshelve info into devref doc"