Deactivate libvirtVM.Drives()

Change I8d310f216987b7 has introduced teardownVolumePath() calls with a
Drive object instead of a drive dictionary from self.conf.
In its previous form, teardownVolumePath() has silently ignored such
input, keeping the volume active.

This patch makes teardownVolumePath() handle such inputs properly. This
is one baby step in the process of getting rid of self.conf: we would
like to use the objects everywhere within vdsm.

Note that this patch introduces an ugly log line when pointlessly trying
to tear down a cdrom device. This would be fixed in a subsequent patch.

Change-Id: I8ba965f0987037fa5dafc6b3ea6eeb6f01a82436
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=962549
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14869
Reviewed-on: http://gerrit.ovirt.org/14953
Tested-by: Federico Simoncelli <fsimonce@redhat.com>