vm: sampling: docs: document expected data format

Add docstrings to sample functions to document
the expected output.

It is hopefully more practical to keep this
doc up to date instead of digging into the libvirt
python bindings to rediscover the wheel each time.

Change-Id: I5aafe1c7b34a3e96123164575b956de9bc13cbf4
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36106
Reviewed-by: Dan Kenigsberg <danken@redhat.com>