There is a much simpler method for making &/or editing existing boot images.
Search for a freeware utility called file called vfd.exe or vfdwin.exe This is an extremely light weight virtual floppy device driver. I tried to find
You certainly don't need a virtual machine & the additional memory to run it.
Download it here...
http://down2.zol.com.cn/xitongruanjian/vfloppy.rar