virsh.nodedev-reset: Tighten unresettable devices

Defining devices `unresettable` as "any of paths `reset` or `devices` are
missing" is not always true, since some resettable device don't have
`devices` path exist under sysfs like a memory controller.

This patch tighten the definition `unresettable` devices to neither two
paths are found, while the definition of `resettable` devices not
changed.

This patch also update the libvirtd control code with the new Libvirtd
class. The original libvirtd status backup feature is removed since the
script shouldn't be called if libvirtd is stopped.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #396 from Hao-Liu/fix_nodedev_reset

virsh.nodedev-reset: Tighten unresettable devices