It is a security best practice recommendation to ensure that all your VMs are fully patched before they are turned on in production.
One way you can do this is to create a designated "maintenance host" that is off the production network, but has access to the needed software updates. Migrate your VMs to the maintenance host, turn them on, patch them up, then migrate them to production. Virtual Machine Manager makes this easy.
Another way to accomplish this is to patch all your offline or stored VMs at the same time as you patch all your running machines. You can use the free Offline Virtual Machine Servicing Tool for this.
Download the Offline Virtual Machine Servicing Tool, which combines the Windows Workflow programming model with the Windows PowerShell interface to bring groups of virtual machines online just long enough for them to receive updates from either System Center Configuration Manager 2007 or Windows Server Update Services. As soon as the virtual machines are up-to-date, the tool returns them to the offline state in the Virtual Machine Manager library.
The download includes: