Description of problem:

I thought that 'fat' would be a valid filesystem type, but
apparently it's not:

$ virt-make-fs -t fat --partition=mbr libguestfs-1.18.0.tar.gz disk.img
Formatting 'disk.img', fmt=raw size=44308070 
mkfs: fat: /dev/vda1: mkfs.fat: No such file or directory at /usr/bin/virt-make-fs line 459.

(-t vfat works).  I think the error message could be improved.
Could we also have a way to detect what filesystems are supported
by various operations (eg. mount, mkfs, etc?)

Version-Release number of selected component (if applicable):

1.18.0.

How reproducible:

100%

Steps to Reproduce:
1. As above.