Set raw image size to be multiple of 64k

This works around a bug in qemu-img causing it to sometimes error if
compressing a raw image that isn't a multiple of 64k (the qcow2
cluster size).
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1180021
http://lists.gnu.org/archive/html/qemu-devel/2013-04/msg02520.html

Change-Id: I88f8bb5c6bf3606ef5856b085a284af6b8858dbb