Benchmark for new preallocate filter.

Example usage:
    ./bench_prealloc.py ../../build/qemu-img \
        ssd-ext4:/path/to/mount/point \
        ssd-xfs:/path2 hdd-ext4:/path3 hdd-xfs:/path4

The benchmark shows performance improvement (or degradation) when use
new preallocate filter with qcow2 image.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201021145859.11201-22-vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>