From the screenshot of clonedisk you posted it seems like the volume on it was not given a drive letter, maybe Rufus uses the latter to detect the volume (and "hosting disk" )
The vhd is assigned a volume label which can been seen in XP's Explorer and Disk Manager, Clone Disk clears the Logical Drive field when taking a screenshot via the Tools menu.
About "detecting" a fixed disk image (under the assumption that "if it is not fixed, then it is something else" ) you can simply open in a disk editor the last sector of the image.
If it is the Conectix "footer" it is "fixed".
Or check the first sector, if it is NOT a MBR, then it is "not fixed".
Using a file hex editor, the string "AKEO" pertaining to Rufus can be been seen in the header and the
"conectix" string in the footer, so that conflictingly suggests(to me at least), that it is both fixed
and not fixed at the same time. I'll try deleting the conectix sector to covert the file from RAW to VHD to see if this helps me.
BUT, on second thought, wait a minute, what is actually meant by "Rufus does not detect"?
AFAICU/AFAICR Rufus ONLY "detects" USB devices intended as "target".
An image mounted through one of these drivers won't be attached to a USB bus (real or virtual).
Creating a bootable virtual disk in Rufus can be achieved in Windows 7 at least, by clicking on the "Format Options" button and then enabling the "List USB Hard Drives" checkbox under Advanced Settings.