Use sparse files instead of files written full of zeros. This speeds up
the image generation process significantly. It also makes sure that
virtual disk image formats are minimal in size.
Note: qemu-img automatically generates sparse files when detecting a
block full of zeros. But this is applied on the write side, after image
convertion: The disk image format itself still thinks the whole image
is allocated, leading to larger image than necessary. Also some output
format seem to regonize chunks of zero and create sparse files themself.
With this change, the raw source image file is a sparse file. This is
regocnized by qemu-img at read time (see block/file-posix.c), and leads
to "native" sparse files in the output format.
Some numbers
- qcow2 1.8G -> 862M (same on-disk size)
- vdi 15G -> 888M (same on-disk size)
- vhdx 30G -> 1.1G (918M -> 861M on-disk size)
- vmdk 1.8G -> 866M (about the same on-disk size)
Obviously this also affects the compressed size. But because there are
still lots of zeros, the difference in compressed size is not that big.
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