The imagecache related code in libvirt.utils uses the base_dir_name
config option.
If we move the imagecache utils into imagecache, then we can move
the base_dir_name option into that module since there are no other
users of it.
Having all the imagecache code in the imagecache module seems like
a sensible thing to do anyway.
blueprint: scope-config-opts
Change-Id: I1e154aa4de1628d40964207bdcbe5b3b55076442