CA-150395: Ignore Storage_interface.Sr_not_attached exception when do xapi_vbd.unplug

Code fix for [SCTX-1901]Some VDIs can't be unplugged in Primary Storage when executing many volume snapshot jobs from CCP
like SCTX-1892, but seems that we cannot catch Storage_interface.Sr_not_attached exception at storage_impl.ml
Here're a possible solution: use try-with statement to wrap xapi_vbd.unplug