Shelve_offload() should give guests a chance to shutdown

Change Iec3dfd17725440958aac395ebc471e51afd6522e added clean_shutdown
support to shelve operations, but ignored shelve_offload as its normally called
either as a second stage to shelve().   However BFV instances call
shelve_offload() directly, so it too should have clean_shutdown support.

Change-Id: Ida612a70ad3b8a7b7b9454579870569c4129ee48
Closes-Bug: 1384637
Partially-Implements: blueprint user-defined-shutdown
Merge "Shelve_offload() should give guests a chance to shutdown"