Qemu: prevent possible buffer overflows

Multiple buffer overflows in QEMU before 1.7.2 and 2.x before 2.0.0, allow local users to cause a denial of service crash or possibly execute arbitrary code via a large 1 L1 table in the qcow2snapshotloadtmp in the QCOW 2 block driver block/qcow2-snapshot.c or 2 uncompressed chunk, 3 chunk length...