Fix identification of blockdev block size
Fix identification of blockdev block size

It was likely a typo to try accessing a "partition" directory under
/sys/block/, where a "disk" directory was intended.

Signed-off-by: Yann Dirson <9a3266eb84fe75b6e4a0d9d8e42ed70b568ec175@vates.fr>

xenserver/xe-guest-utilities,xenserver/xe-guest-utilities,xenserver/xe-guest-utilities