qcow2: Allow qcow2 disk images with size zero

qcow2: Allow qcow2 disk images with size zero

Images with disk size 0 may be used for
VM snapshots, but not to save normal block data.

It is possible to create such images using
qemu-img, but opening them later fails.

So even "qemu-img info image.qcow2" is not
possible for an image created with
"qemu-img create -f qcow2 image.qcow2 0".

This is fixed here.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>