ceph-volume: enhance BlockSysFs

ceph-volume: enhance BlockSysFs

This refactores `BlockSysFs` to introduce `_get_sysfs_file_content()` for
retrieving sysfs file contents in a cleaner and reusable way.

It renames `self.sys_dev_block` to `self.sys_dev_block_dir` for clarity.
I've added `_get_sysfs_file_content()` to centralize sysfs file reads.

3 new methodes are introduced:
`blocks`, `logical_block_size`, and `size` properties for computing block
device size in a structured manner.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>