CA-107572: Ignore UUID not found exception during SR scan

VM migration failed because XAPI throw VDI UUID not found exception.
The VDI was already removed, the fix will catch the exception and discard it.

Signed-off-by: Sisimon Soman <sisimon.soman@citrix.com>
Acked-by: Thanos Makatos <thanos.makatos@citrix.com

GitHub: closes #17 on xapi-project/sm