"sparse" block devices

for files on supporting filesystems, we have the SEEK_HOLE/SEEK_DATA api.

but this does not apply to block devices, we need something else to detect unused blocks in these.

For block devices, sometimes another API is supported:
- nbd: BLOCK_STATUS, see e.g.: https://github.com/xapi-project/vhd-tool/blob/master/scripts/get_nbd_extents.py
- (add other APIs)

Since recently, borg fixed blocksize chunker can accept a sparse map / file map, but there is no way yet to build that map for a block device.

Related to #14.