Split up devices.py.

This did not change any code except for imports within blivet/devices/.

Unit tests pass, too (as non-root -- didn't try as root yet).

The bad news is that 'git blame' is pretty badly crippled. For example, blivet/devices/disk.py blames almost every line on the commit that created it, which means it lost the blame history for all that code. Oddly, blivet/devices/btrfs.py has its blame info completely intact.

I'm concerned about the git blame issue, but this seems like such a nice improvement I almost think it would be worthwhile anyway. I'm putting this out to see what others think.

BEFORE:
[david@localhost blivet]$ ls -l blivet/devices.py
-rw-rw-r--. 1 david david 200502 Oct  2 17:57 blivet/devices.py

AFTER:
[david@localhost blivet]$ ls -l blivet/devices/
total 256
-rw-rw-r--. 1 david david 23011 Oct  2 17:49 btrfs.py
-rw-rw-r--. 1 david david  5742 Oct  2 17:50 container.py
-rw-rw-r--. 1 david david 10505 Oct  2 17:50 device.py
-rw-rw-r--. 1 david david 21581 Oct  2 17:50 disk.py
-rw-rw-r--. 1 david david  8845 Oct  2 17:50 dm.py
-rw-rw-r--. 1 david david  4930 Oct  2 17:50 file.py
-rw-rw-r--. 1 david david  1969 Oct  2 17:50 __init__.py
-rw-rw-r--. 1 david david  5251 Oct  2 17:49 lib.py
-rw-rw-r--. 1 david david  4280 Oct  2 17:50 loop.py
-rw-rw-r--. 1 david david  3441 Oct  2 17:50 luks.py
-rw-rw-r--. 1 david david 44993 Oct  2 17:50 lvm.py
-rw-rw-r--. 1 david david 22083 Oct  2 17:50 md.py
-rw-rw-r--. 1 david david  1997 Oct  2 17:49 network.py
-rw-rw-r--. 1 david david  2938 Oct  2 17:51 nfs.py
-rw-rw-r--. 1 david david  3573 Oct  2 17:51 nodev.py
-rw-rw-r--. 1 david david  2864 Oct  2 17:51 optical.py
-rw-rw-r--. 1 david david 29039 Oct  2 17:51 partition.py
-rw-rw-r--. 1 david david 25593 Oct  2 17:51 storage.py