Log cleanups for nova.virt.libvirt.imagecache

More log cleanups:
 - add log hints for error, warning and info levels
 - move from LOG.warning to LOG.warn

Change-Id: I452cf18f9a5199bfc1c1b104f6a2b03561bf2fc4
Merge "Log cleanups for nova.virt.libvirt.imagecache"