INT 2F - DESQview v2.26+ External Device Interface - INSTALLATION CHECK
        AX = DE00h
        BX = 4445h ("DE")
        CX = 5844h ("XD")
        DX = 4931h ("I1")
Return: AL = FFh if installed (even if other registers do not match)
        if BX,CX, and DX were as specified on entry,
            BX = 4845h ("HE")
            CX = 5245h ("RE")
            DX = 4456h ("DV")
Notes:  AH=DEh is the default XDI multiplex number, but may range from C0h-FFh
        programs should check for XDI starting at DEh to FFh, then C0h to DDh
        the XDI handler should not issue any DOS or BIOS calls, nor should it
          issue DESQview API calls other than those allowed from hardware ints
SeeAlso: INT 15/AX=5400h