virsh: Print cephx and iscsi usage

virsh: Print cephx and iscsi usage

When using virsh secret-list - if the secret types are cephx or iscsi,
then allow fetch/print of the usage information. Prior to the change
the following would print:

UUID                                 Usage
-----------------------------------------------------------
1b40a534-8301-45d5-b1aa-11894ebb1735 Unused
a5ba3efe-6adf-4a6a-b243-f010a043e314 Unused

Afterwards:

UUID                                 Usage
-----------------------------------------------------------
1b40a534-8301-45d5-b1aa-11894ebb1735 ceph ceph_example
a5ba3efe-6adf-4a6a-b243-f010a043e314 iscsi libvirtiscsi