Change method names in vmtune.py to the PEP8 style

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120246
Change-Id: Ib5e9dcaf393ebafe56f6bc78cc9cb02d37a257ae
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29929
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30078
Reviewed-by: Francesco Romani <fromani@redhat.com>