switch: resolve device path so we can umount the osd data dir

switch: resolve device path so we can umount the osd data dir

If we don't do this, umounting devices declared like this
/dev/disk/by-id/ata-QEMU_HARDDISK_QM00001

will fail like:

umount: /dev/disk/by-id/ata-QEMU_HARDDISK_QM000011: mountpoint not found

Since we append '1' (partition 1), this won't work.
So we need to resolved the link to get something like /dev/sdb and then
append 1 to /dev/sdb1

Signed-off-by: Sébastien Han <seb@redhat.com>
Co-authored-by: Guillaume Abrioux <gabrioux@redhat.com>