Move flags in libvirt's volume 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 have moved into the libvirt group:
    num_iscsi_scan_tries
    num_iser_scan_tries
    rbd_user
    rbd_secret_uuid
    nfs_mount_point_base
    nfs_mount_options
    num_aoe_discover_tries
    glusterfs_mount_point_base
    iscsi_use_multipath (formerly libvirt_iscsi_use_multipath)
    iser_use_multipath (formerly libvirt_iser_use_multipath)
    scality_sofs_config
    scality_sofs_mount_point
    qemu_allowed_storage_drivers

Change-Id: I78941aff4bf49fc70d91a3239deff44dbaaab6c9
Merge "Move flags in libvirt's volume to the libvirt group"