An instance libvirtError will add resources to the blacklist

[dejianfu]

Libvirterror occurs when ceilometer polls cpu or memory metrics,
The poll_and_notify method captures the PollerPermanentError and adds resources to the blacklist.
Even if libvirt returns to normal, it will not clean up blacklist

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Tristan Zhang (tzmtl)]

We met the same issue. Sometimes ceilometer gets an exception caused by libvirt,

For example,

1. qemuMonitorIORead:610 : Unable to read from monitor: Connection reset by peer
2. virNetSocketReadWire:1811 : End of file while reading data: Input/output error

Most time these are just random and temporary issues, not fatal and permanent. If ceilometer tries again the next minute, it can get data from libvirt. But since ceilometer add the group of resources to blacklist, it will not poll data from libvirt anymore. The only way to make it work again is to restart ceilometer-compute agent.

Below is the code causing issues,

ceilometer/polling/manager.py

```
except plugin_base.PollsterPermanentError as err:
    LOG.error(
        'Prevent pollster %(name)s from '
        'polling %(res_list)s on source %(source)s anymore!',
        dict(name=pollster.name,
             res_list=str(err.fail_res_list),
             source=source_name))
    self.resources[key].blacklist.extend(err.fail_res_list)
```