qcow2: avoid misaligned 64bit bswap

If we create a buffer directly on the stack by using 12 bytes, there's
no guarantee the 64bit value we want to swap will be aligned, which
could cause errors with undefined behavior.

Spotted with clang -fsanitize=undefined and observed in iotests 15, 26,
44, 115 and 121.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
