Fix Subvolume permissions issues

[cezarmathe]

The Subvolume struct should include the id, the filesystem root and the path.

Create two ways of getting a Subvolume:

• one that requires the exact path of the subvolume(will not require CAP_SYS_ADMIN)
• one that will retrieve the subvolume at a certain path(will require CAP_SYS_ADMIN)

Refactor all functions that require the path so that they do not call the C function every time.

[cezarmathe]

This should fix #1

[cezarmathe]

Fixed by #14