storage: Add capability to use LUKS encryption for disk backend
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1560946
    
    Similar to the the Logical backend, use qemu-img on the created
    disk partition device to set up for LUKS encryption. Secret mgmt
    for the device can be complicated by a reboot possibly changing
    the path to the device if the infrastructure changes.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>
    ACKed-by: Peter Krempa <pkrempa@redhat.com>