Introducing qcow2 extensions (Uri Lublin)

Introducing qcow2 extensions (Uri Lublin)

Qcow2 extensions are build of magic (id) len (in bytes) and data.
They reside right after the qcow2 header.
If a backing filename exists it follows the qcow2 extension (if exist)

Qcow2 extensions are read upon image open.
Qcow2 extensions are identified by their magic.
Unknown qcow2 extensions (unknown magic) are skipped.
A Special magic of 0 means end-of-qcow2-extensions.

In this patchset, to be used to keep backing file format.

Based on a work done by Shahar Frank <sfrank@redhat.com>.

Signed-off-by: Uri Lublin <uril@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6907 c046a42c-6fe2-441c-8c8c-71466251a162