vdsm: remove protected attribute access in VmStatsThread

Accessing internal elements is evil, VmStatsThread accessed
vm devices through _vm._devices[DISK_DEVICES], which can be
replaced by _vm.getDiskDevices()

Change-Id: I79736cf6dfa18a251ad050a7fd51b94339a0958b
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32246
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>