kvm: Add better logging when fetching a volume from libvirt

Clearly show if a volume is found and if not, that the pool is being refreshed
and the fetch is tried again.

Due to my commit b53a9dcc9f3ee95d40761b9c2c860f821595a661 the chance of a volume
not being found is slightly bigger, but the performance gain is enormous on larger
deployments.

This is why we clearly have to log that we are refreshing the pool information
when a volume is not found.

It could be that a volume is created on host A and a few seconds later host B tries
to access the volume. In that case host B's libvirt doesn't know about the volume
yet and has to refresh the pool before it does.