Edit Virtual Machine Startup and Shutdown Settings

You can configure virtual machines running on an ESXi host to start up and shut down with the host. You can also set the default timing and startup order for selected virtual machines. This ability allows the operating system to save data when the host enters maintenance mode or is being powered off for another reason. This setting is disabled when DRS cluster is enabled.

Procedure

1. In the vSphere Client inventory, select the host where the virtual machine is located and click the Configuration tab.

2. Under Software, click Virtual Machine Startup/Shutdown and click Properties.

The Virtual Machine Startup and Shutdown dialog box opens.

3. Select Allow virtual machines to start and stop automatically with the system.

Sources : http://vcp5.wordpress.com/2012/01/22/edit-virtual-machine-startup-and-shutdown-settings/
http://deinoscloud.wordpress.com/2010/05/17/vsphere-virtual-machine-startup-and-shutdown-behavior/