Support for XDF disk format

[rderooy]

Is your feature request related to a problem? Please describe.
Some IBM software, most notably PC-DOS 7.0/2000 and OS/2 Warp 3/4 where distributed on XDF formatted 3.5" and 5.25" HD disks.

Describe the solution you'd like
Support for XDF format for IMGMOUNT and BOOT.
Note: XDF formatted disks are not bootable, but you could have it as a second disk.
e.g
BOOT dos7.img disk.xdf

Describe alternatives you've considered
It should be possible to install PC-DOS 2000 from CD-ROM in dosbox (have not yet tried), using a regular (non-XDF) boot disk.

Additional context
https://en.wikipedia.org/wiki/IBM_Extended_Density_Format
http://www.os2museum.com/wp/the-xdf-diskette-format/
https://github.com/claunia/CreateXDF
https://sourceforge.net/p/sevenzip/discussion/45797/thread/1e0493b340/?limit=25
keirf/flashfloppy#54

Note: In the above FlashFloppy github issue, there is some talk about 'ED" (2.88) disks in XDF format. I was an IBM employee at the time, and had contacts in software development. I used lots of XDF format disks. By the time XDF came out, ED disks where well and truly dead, even at IBM. As such, I don't think ED XDF disks ever existed.