migration: introduce SaveVMHandlers.resume_prepare

This is hook function to be called when a postcopy migration wants to
resume from a failure. For each module, it should provide its own
recovery logic before we switch to the postcopy-active state.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180502104740.12123-16-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
5 files changed