qcow2: include LUKS payload overhead in qemu-img measure

qcow2: include LUKS payload overhead in qemu-img measure

LUKS encryption reserves clusters for its own payload data.  The size of
this area must be included in the qemu-img measure calculation so that
we arrive at the correct minimum required image size.

(Ab)use the qcrypto_block_create() API to determine the payload
overhead.  We discard the payload data that qcrypto thinks will be
written to the image.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 20190218104525[email protected]
Signed-off-by: Max Reitz <[email protected]>