Move flags in libvirt's imagebackend 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:
    libvirt_images_type (now images_type)
    images_volume_group
    sparse_logical_volumes
    lvm_snapshot_types
    images_rbd_pool
    images_rbd_cepf_conf

Change-Id: Ic1bb8076b139787fe5acbc0574b0c75b52bf3bd7
Merge "Move flags in libvirt's imagebackend to the libvirt group"