virt plugin: Properly reset the lists

virt plugin: Properly reset the lists

If all the libvirt domains are shutdown between two refresh cycles,
the code failed to clean the lists.
Thus, the last detected libvirt domains are kept forever, leading
to failures in the data collection.
This leads to messages like

Nov 24 15:34:57 benji.rokugan.lan collectd[15083]: libvirt: QEMU Driver
error : Domain not found: no domain with matching uuid
'bc324fee-5707-4ac6-a650-6037562d4632' (nano_C010)

In the logs.
This bug was also present in collectd 5.6.1.

Signed-off-by: Francesco Romani <fromani@redhat.com>