CA-106886: Don't update vhd-blocks during SR.scan

Don't bother updating the vhd-blocks in the XAPI database as this is
owned by the GC and there's no synchronisation for this.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>
Reviewed-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Imported-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #119 on xapi-project/sm
(cherry picked from commit 083722c17dcc8a29371ae2a4fd00c6a87ce3275d)