block: export LUKS specific data to qemu-img info

The qemu-img info command has the ability to expose format
specific metadata about volumes. Wire up this facility for
the LUKS driver to report on cipher configuration and key
slot usage.

    $ qemu-img info ~/VirtualMachines/demo.luks
    image: /home/berrange/VirtualMachines/demo.luks
    file format: luks
    virtual size: 98M (102760448 bytes)
    disk size: 100M
    encrypted: yes
    Format specific information:
        ivgen alg: plain64
        hash alg: sha1
        cipher alg: aes-128
        uuid: 6ddee74b-3a22-408c-8909-6789d4fa2594
        cipher mode: xts
        slots:
            [0]:
                active: true
                iters: 572706
                key offset: 4096
                stripes: 4000
            [1]:
                active: false
                key offset: 135168
            [2]:
                active: false
                key offset: 266240
            [3]:
                active: false
                key offset: 397312
            [4]:
                active: false
                key offset: 528384
            [5]:
                active: false
                key offset: 659456
            [6]:
                active: false
                key offset: 790528
            [7]:
                active: false
                key offset: 921600
        payload offset: 2097152
        master key iters: 142375

One somewhat undesirable artifact is that the data fields are
printed out in (apparently) random order. This will be addressed
later by changing the way the block layer pretty-prints the
image specific data.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1469192015-16487-3-git-send-email-berrange@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
