Specification for qcow2 version 3

This updates the qcow2 specification to cover version 3. It contains the
following changes:

- Added compatible/incompatible/auto-clear feature bits plus an optional
  feature name table to allow useful error messages even if an older
  version doesn't know some feature at all.

- Configurable refcount width. If you don't want to use internal
  snapshots, make refcounts one bit and save cache space and I/O.

- Zero cluster flags. This allows discard even with a backing file that
  doesn't contain zeros. It is also useful for copy-on-read/image
  streaming, as you'll want to keep sparseness without accessing the
  remote image for an unallocated cluster all the time.

- Fixed internal snapshot metadata to use 64 bit VM state size. You
  can't save a snapshot of a VM with >= 4 GB RAM today.

- Extended internal snapshot metadata to contain the disk size, so that
  resizing images that have snapshots can be allowed in the future.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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