[Qemu-devel] [RESEND][PATCH For Review 0/2] qemu block changes: keep backing file format v3

[Uri Lublin]

This is another implementation for keeping a backing file format, such
that we would not need probing to figure it out.

This time I introduce qcow2 extensions which can be used in the
future.
I using one such an extension to keeping the backing file format,
and use it when opening a file.

Qcow2 extensions are build of magic (id) len (in bytes) and data.
They reside between the end of the header and the filename.

We keep the backing file format in a such a qcow2 extension, to
1. Provide a way to know the backing file format without probing
   it (setting the format at creation time).
2. Enable using qcow2 format over host block devices.
   (only if the user specifically asks for it, by providing the format
   at creation time).

I've added bdrv_create2 and drv->bdrv_create2 (implemented only
by block-qcow2 currently) to pass the backing-format to create.

Based on a work done by Shahar Frank.

Also fixes a security flaw found by Daniel P. Berrange on [1]
which summarizes: "Autoprobing: just say no."

[1] http://lists.gnu.org/archive/html/qemu-devel/2008-12/msg01083.html

First patch implements qcow2 extensions + bdrv_create2
Second patch modifies qemu-img such that the user can use it.

Currently this is implemented only for qcow2, but touches code in some
block*.[ch] files. An alternative that touches only block-qcow2.c is
possible, but will need to change the implementation of -snapshot,
and call bdrv_open2 directly from qcow_open (to open the backing file).

Please review,
Thanks,
    Uri.