Supply an API for creating a VM without starting it

In the "create server" API, allow the user to supply a boolean value which states whether the VM should be powered on upon creation or not.
Of course that for backward-compatibility, the default should be to power on a VM upon creation.
Example use-cases:
* a user wants to set up a cloud fast, and powering up VMs is too much of an overhead for him
* an admin has an image that includes configuring on the first boot, and he wants to allow another teammate to do the first and to configure the machine.