Fix getting properties

[Apostoln]

• Use syspath instead of sysname for udev
• Add dev_path_to_sys_path() function
• Return error if there no sys path

Close #22

There was a bug with incorrect syspath for udev. I've fixed it making an assumption we work only with block devices (and return the Error otherwise) and the syspath is always like /sys/class/block/.

I've tested it locally and with the integration tests in our project where we use block-utils. However, it's a bit complicated to write unit tests in the scope of this repo because it requires a kind of testing framework for loop devices, disks, partitions, filesystems, and so on. We have a kind of such one in progress, but it hasn't opensourced yet.

Well, I'm not sure by 100% that the assumption about /sys/class/block is general truth and this path exists for all block devices. There are a lot of other sys paths like /sys/devices/virtual/block/ and /sys/block/. Anyway, it looks like /sys/class/block is an expected syspath according to the examples in udev.

[Apostoln]

I've found some potential problems, please don't merge now.

[Apostoln]

Doesn't matter, it were unrelated problems with Fedora 33.

[cholcombe973]

This PR looks reasonable to me. Would you like me to merge it?

[Apostoln]

@cholcombe973 Yes, thank you :)