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>
Currently It will fail out in the preprocess phase when running a libvirt test case without libvirt daemon started. Although not very intuitive, this should keep any of the test module from running. So I think we don't need to check the libvirtd daemon status in the test module. What's your opinion on this @yumingfei ?
```
12:56:45 ERROR| Traceback (most recent call last):
12:56:45 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/standalone_test.py", line 210, in run_once
12:56:45 ERROR|     params = env_process.preprocess(self, params, env)
12:56:45 ERROR|   File "/home/celeron/Workspace/virt-test/autotest/client/shared/error.py", line 141, in new_fn
12:56:45 ERROR|     return fn(*args, **kwargs)
12:56:45 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/env_process.py", line 699, in preprocess
12:56:45 ERROR|     connect_uri = libvirt_vm.normalize_connect_uri(connect_uri)
12:56:45 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/libvirt_vm.py", line 42, in normalize_connect_uri
12:56:45 ERROR|     raise ValueError("Normalizing connect_uri '%s' failed" % connect_uri)
12:56:45 ERROR| ValueError: Normalizing connect_uri 'default' failed
12:56:45 ERROR| 
12:56:45 ERROR| FAIL type_specific.io-github-autotest-libvirt.virsh.nodedev_reset.negative_test.unresettable_device -> ValueError: Normalizing connect_uri 'default' failed
```