migration: unbreak postcopy recovery

migration: unbreak postcopy recovery

The whole postcopy recovery logic was accidentally broken.  We need to
fix it in two steps.

This is the first step that we should do the recovery when needed.  It
was bypassed before after commit 36c2f8be2c.

Introduce postcopy_try_recovery() helper for the postcopy recovery
logic.  Call it both in migration_fd_process_incoming() and
migration_ioc_process_incoming().

Fixes: 36c2f8be2c ("migration: Delay start of migration main routines")
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180627132246.5576-4-peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>