For raw, this allows "off" as a synonym for "sparse" (to make it
consistent with qcow2).
For qcow2, this allows "sparse" as a synonym for "off".
It also adds qcow2 "full" preallocation, which is actually mapped to
the qemu option "falloc" (see arguments about this on the qemu-devel
mailing list, which we lost).
This also updates the test.