Added support for multiple sessions in the interactive console.
The interactive session now shows the session name by default instead of the image name. Set the session name with smod session_name="New name".
New plugin: slist (lists the available sessions)
New plugin: snew (clones the current session)
New plugin: smod (changes parameters of the current session)
New plugin: sdel ID (deletes a session by ID)
New plugin: sswitch ID (switches the current active session to the session with ID)
vmscan: Fixed a bug where VMCSs without EPT would make it raise.
vmscan: Added a new flag --quick, to stop validation of a VM when 1 VMCS associated to it is found to be valid. Improves performance at the expense of not counting vCOREs.
vmscan: Added a new flag --image_is_guest to properly validate VMs when analyzing guest memory directly (i.e nested virtualization and when analyzing a .vmem file).
vmscan: Switched to tree rendering.
vmscan: Now adds a session for each virtual machine found valid while running from the interactive session.
Added renderers for VirtualMachine and JSON rendering for VTxPagedMemory.

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Review URL: https://codereview.appspot.com/136130043