CA-105130: udev does not automatically refresh dev/disk/by-id

udev maintains symlinks into /dev/disk/by-id refreshing to
the latest available path to the same disk.
When the last added path is removed, the link is removed as
well, instead of being just updated to the previous path.

Recent versions of udev (e.g. in CentOS 6.4) do not show
this behaviour but our current version (095) needs to be triggered.

Signed-off-by: Germano Percossi <germano.percossi@citrix.com>
Acked-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #19 on xapi-project/sm