blocks to-bcache uses wrong device

[krichter722]

invoking sudo blocks to-bcache /dev/sda5 causes output

Will shrink the filesystem (btrfs) by 1048576 bytes
Command ['mount', '-t', 'btrfs', '-o', 'noatime,noexec,nodev', '--', '/dev/sda4', '/tmp/tmp8uo886hz.privmnt'] has failed with status 32
Standard output:

Standard error:
mount: /dev/sda4 is already mounted or /tmp/tmp8uo886hz.privmnt busy

Traceback (most recent call last):
  File "/usr/local/bin/blocks", line 9, in <module>
    load_entry_point('blocks==0.1.4', 'console_scripts', 'blocks')()
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 2012, in script_main
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 1678, in main
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 1761, in cmd_to_bcache
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 1560, in part_to_bcache
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 425, in reserve_space_before
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 413, in _reserve_range
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 1096, in stack_reserve_end_area
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 501, in reserve_end_area_nonrec
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 545, in _mount_and_resize
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 883, in _resize
  File "/usr/lib/python3.4/contextlib.py", line 59, in __enter__
    return next(self.gen)
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 513, in temp_mount
  File "/usr/local/lib/python3.4/dist-packages/blocks-0.1.4-py3.4.egg/blocks/__main__.py", line 110, in quiet_call
subprocess.CalledProcessError: Command '['mount', '-t', 'btrfs', '-o', 'noatime,noexec,nodev', '--', '/dev/sda4', '/tmp/tmp8uo886hz.privmnt']' returned non-zero exit status 32

which clearly indicates that /dev/sda4 is used instead of /dev/sda5.

[krichter722]

By accident I figured out how blocks work. You definitely need to explain it in README.