block/cloop: validate block_size header field (CVE-2014-0144)

block/cloop: validate block_size header field (CVE-2014-0144)

Avoid unbounded s->uncompressed_block memory allocation by checking that
the block_size header field has a reasonable value.  Also enforce the
assumption that the value is a non-zero multiple of 512.

These constraints conform to cloop 2.639's code so we accept existing
image files.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>