esp: recreate ESPState current_req after migration

Since PDMA reads/writes are driven by the guest, it is possible that migration
can occur whilst a SCSIRequest is still active. Fortunately active SCSIRequests
are already included in the migration stream and restarted post migration but
this still leaves the reference in ESPState uninitialised.

Implement the SCSIBusInfo .load_request callback to obtain a reference to the
currently active SCSIRequest and use it to recreate ESPState current_req
after migration.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220305155530.9265-11-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>