qcow2: add support for LUKS encryption format

This adds support for using LUKS as an encryption format
with the qcow2 file, using the new encrypt.format parameter
to request "luks" format. e.g.

  # qemu-img create --object secret,data=123456,id=sec0 \
       -f qcow2 -o encrypt.format=luks,encrypt.key-secret=sec0 \
       test.qcow2 10G

The legacy "encryption=on" parameter still results in
creation of the old qcow2 AES format (and is equivalent
to the new 'encryption-format=aes'). e.g. the following are
equivalent:

  # qemu-img create --object secret,data=123456,id=sec0 \
       -f qcow2 -o encryption=on,encrypt.key-secret=sec0 \
       test.qcow2 10G

 # qemu-img create --object secret,data=123456,id=sec0 \
       -f qcow2 -o encryption-format=aes,encrypt.key-secret=sec0 \
       test.qcow2 10G

With the LUKS format it is necessary to store the LUKS
partition header and key material in the QCow2 file. This
data can be many MB in size, so cannot go into the QCow2
header region directly. Thus the spec defines a FDE
(Full Disk Encryption) header extension that specifies
the offset of a set of clusters to hold the FDE headers,
as well as the length of that region. The LUKS header is
thus stored in these extra allocated clusters before the
main image payload.

Aside from all the cryptographic differences implied by
use of the LUKS format, there is one further key difference
between the use of legacy AES and LUKS encryption in qcow2.
For LUKS, the initialiazation vectors are generated using
the host physical sector as the input, rather than the
guest virtual sector. This guarantees unique initialization
vectors for all sectors when qcow2 internal snapshots are
used, thus giving stronger protection against watermarking
attacks.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-14-berrange@redhat.com
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
