qcow2: add bitmaps extension

qcow2: add bitmaps extension

Add bitmap extension as specified in docs/specs/qcow2.txt.
For now, just mirror extension header into Qcow2 state and check
constraints. Also, calculate refcounts for qcow2 bitmaps, to not break
qemu-img check.

For now, disable image resize if it has bitmaps. It will be fixed later.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: John Snow <[email protected]>
Message-id: 20170628120530[email protected]
Signed-off-by: Max Reitz <[email protected]>