Move flag in libvirt's utils 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 flag is now in the libvirt group:
    snapshot_compression (formerly libvirt_snapshot_compression)

Change-Id: I9eddec5cdb72a1313e044a3c7c6d75e8ad34533a
Merge "Move flag in libvirt's utils to the libvirt group"