After reviewing a variety of different virtualization products there is only 1 constant. They all happily boot from an ISO cdrom image. Each vendor has their own format for the hard drive, and there are a variety of floppy disk image formats also.

This is a draft, which as time allows I will expand

What I have done is gather the list to make a simple table outlining the floppy image formats, and the descriptions of them:

.dsk

There are a number of programs that produce images of disks. These images typically start with hex EB and, as an image, most any program that can produce/read a disk image can read the image produced by a different program. Various programs use this extension; too many to list individually. Take clues from the location of the file as a possible pointer to exactly which program is producing the file. The file’s date and time can also help if you know which programs you were running when the file was written. (source)

.img

All image creates image files of any disk (USB Flash Drives, Floppy Drives, Hard Drives, etc). Saves the image file compressed, uncompressed or as a highly customizable self-extracting EXE using a wizard-like interface.

Note: QEMU is a generic and open source processor emulator which achieves emulation speed by using dynamic translation. Qcow is a QEMU specific image format, with support for compression and optional AES encryption. (source)

.ffd

.vfd

.dmg

On the Macintosh, these files are treated like a real disk. They can be created with Disk Copy, burnt to CD or mounted as a normal volume. If you have a .DMG file on a Windows PC it’s likely you have a Macintosh file and you won’t be able to use what is in it. Some sites distribute for the PC in an .EXE file and for the Mac in a .DMG file; make certain you have the version designed for your system. On a Windows PC the dmg2iso program will convert a .DMG file to an .ISO file which can then be burned to a CD or read using an .ISO file reader. Note: You have to run the dmg2iso program at a command prompt using the same command as listed on the site for the Perl version, substituting .EXE for .PL in the command. DMGs can be password-protected and, if so, may not be mountable by some software. The DMG file also comes in different formats: HFS, HFS+, UFS, ProDOS, Linux, and Fat32 and so may also require special mounting software to account for the format. IsoBuster can interpret these files directly as can other programs such as UltraISO. (source)

.emt

.iso and .flp
the .flp file is identical to the .iso file (source)

sources:
http://filext.com/file-extension/FLP
http://www.magiciso.com/

Generally notes:

RawWrite is now avail for Windows.
RawWrite (or rawrite) is an essential tool for creating boot disks and other floppy disk images. Traditional rawwrite programs do no run under modern versions of windows so here is the Win32 version which does. (source)

General Sources:
Virtual PC Guy’s WebLog : Floppy disk image formats supported by Virtual PC and Virtual Server
Virtual PC Guy’s WebLog : Using 5.25” floppy disks with Virtual PC
Virtual PC [Archive] - The macosxhints Forums
Rawrite and related programs
emtcopy: Disk Tools
emBoot - Network Boot for Virtual Machines (VM)
VM Back
Virtual PC and Floppy Images : Paco Hope
Gilles Vollant software: winimage
bootdisk.com