virt-df: Disallow -h and --csv options together (RHBZ#600977).

virt-df: Disallow -h and --csv options together (RHBZ#600977).

Before this commit, if you used the -h and --csv options together
you would get these warnings from virt-df:

$ virt-df -h --csv Guest
Virtual Machine,Filesystem,Size,Used,Available,Use%
Argument "13.5G" isn't numeric in printf at /home/rjones/d/libguestfs/tools/virt-df line 298.
Argument "4.7G" isn't numeric in printf at /home/rjones/d/libguestfs/tools/virt-df line 298.
Argument "8.1G" isn't numeric in printf at /home/rjones/d/libguestfs/tools/virt-df line 298.
"/dev/vg_trick/RHEL55x64","/dev/VolGroup00/LogVol00",13,4,8,34.8%
Argument "98.7M" isn't numeric in printf at /home/rjones/d/libguestfs/tools/virt-df line 298.
Argument "18.8M" isn't numeric in printf at /home/rjones/d/libguestfs/tools/virt-df line 298.
Argument "74.9M" isn't numeric in printf at /home/rjones/d/libguestfs/tools/virt-df line 298.
"/dev/vg_trick/RHEL55x64","/dev/vda1",98,18,74,19.0%

We could fix this so that the human-readable numbers get written
into the CSV file.  However would probably be wrong for most uses
of the CSV format (databases and spreadsheets) since they would not
be able to interpret these human-readable numbers, or worse could
misinterpret, eg. thinking that "1M" and "1G" are both 1.

Therefore this commit disallows this combination of options.