To avoid any ambiguities the SLA (not EULA) for Mac OS X Server (Snow Leopard) does not contain the word "retail" in its relevant "Permitted License Uses and Restrictions" section, quoted below although it is in the "7. Limited Warranty on Media." section (the only place it appears). I think the most relevent requirement aside from being used "on the same Apple-branded computer" is "that you acquire an individual and valid license from Apple for each of these other copies of Mac OS X Server Software".
For absolute clarification I'd suggest talking directly with Apple if they'd allow virtualizing an image of a pre-installed copy providing a separate license is purchased. The other issue is whether or not one could actuall install from the Shipped DVD and if not the a Retail DVD may be the only way to install from Optical Media vs. an Image.
SOFTWARE LICENSE AGREEMENT FOR MAC OS X SERVER
...
2. Permitted License Uses and Restrictions.
A. Mac OS X Server Software. Subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install and use one copy of the Mac OS X Server software (the “Mac OS X Server Software”) on a single Apple-branded computer. You may also install and use other copies of Mac OS X Server Software on the same Apple-branded computer, provided that you acquire an individual and valid license from Apple for each of these other copies of Mac OS X Server Software. You agree not to install, use or run the Apple Software on any non-Apple- branded computer, or to enable others to do so. This License does not allow the Mac OS X Server Software to run on more than one computer at a time, and, except as expressly set forth in Section 2C, you may not make the Apple Software available over a network where it could be run by multiple computers at the same time. Please note that the Apple Software is supported only on Apple-branded systems that meet the specified system requirements as indicated on the outside of the Mac OS X Server packaging. If you transfer software from one Apple-branded computer to another Apple-branded computer, please remember that continued use of the original copy of the software may be prohibited once a copy has been transferred to another computer, unless you already have a licensed copy of such software on both computers. You should check the relevant software license agreements for applicable terms and conditions.
You may connect an unlimited number of client computers to each validly licensed copy of the Mac OS X Server Software.