Support virtio disks in graphite iostat model

Signed-off-by: Tim Serong <tserong@suse.com>

Closes #50
Fixed viz tooltip position.

Fixes the position of tooltip that appears on top of the
hostgroups in workbench (sort by host).

Fixes #7778

Signed-off-by: Karnan TC <kchidamb@redhat.com>