libguestfs/df

This is the third rewrite of the virt-df program.  It very much
follows the outline of the Perl program which this replaced in
libguestfs 1.7.14.

main.c -    main program

domains.c - dealing with libvirt, only used if libvirt is around
            at compile time

df.c -      getting the stats from libguestfs

output.c -  writing the output, CSV output

virt-df.h - header file

Note this also uses the shared options parsing code in
'fish/options.[ch]'.