blkdev: overhaul API for better portability

blkdev: overhaul API for better portability

* Turn the API into a class so it will work with GoogleMock
* Take file descriptors for all methods, instead of a mix of file
  descriptors, path names, and canonical device names.  It's more
  consistent and it will work better with FreeBSD.
* Split get_device_by_fd into block_device_partition and
  block_device_wholedisk
* Remove dead code

Signed-off-by: Alan Somers <asomers@gmail.com>