It may seem like a bit of stretch as to why someone would need to boot a VM to anything other than the virtual hard disk, but that’s only because you’ve never run into a situation where you’ve had to do it before. I ran into this problem myself a few weeks ago and I found myself wondering if it’s even possible to do this. On a physical machine you’d simple boot into the BIOS and change the boot order for the system you’re working on. Most newer systems even have a button you can press during startup that will give you a simple boot device selection option as well.
After a bit of research, I found that you can indeed boot a VM from a CD or USB device. As a matter of fact, VM’s have their own BIOS that you can access. It is a bit tricky to access these menus because VM’s typically boot past this screen very quickly, but it can be done. In order to access the boot menu for a VM you must power on the VM and immediately click inside the VM window and tap the escape key as soon as you see the VMWare splash screen. If done in time, you will see a boot device selection prompt, allowing you to boot to an alternate device. Also, pressing F2 during this time period will allow for access to the virtual BIOS, allowing you to permanently change the boot order of the VM.
If you are having trouble accessing these screens due to how small the time window is, you can increase this time window by editing the bios.BootDelay value in the VMX file associated with the VM. This value is in milliseconds and can be increased to as long as you like to ensure you have time to access the BIOS and boot screen menus.