cmdlib: Extract instance operation functionality

Split instance.py further by extracting instance operations
(start/stop/reboot/etc.) related logical units and functions to
instance_operation.py.

The extracted operations have in common that they affect the operating
system in a running instance directly.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>