Move flags in libvirt's imagecache to the libvirt group

This change continues moving libvirt specific flags into a libvirt
specific configuration group.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flags move into the libvirt group:
image_info_filename_pattern, remove_unused_base_images,
remove_unused_kernels, remove_unused_resized_minimum_age_seconds,
remove_unused_original_minimum_age_seconds, checksum_base_images,
checksum_interval_seconds.

Change-Id: Iea681c7db7efc9da0525ef8e01b4900ca71f10cf
Merge "Move flags in libvirt's imagecache to the libvirt group"