mptsas: really fix migration compatibility

Commit 2e2aa316 removed internal flag msi_in_use, but it
existed in vmstate.  Restore it for migration to older QEMU
versions.

Reported-by: Amit Shah <amit.shah@redhat.com>
Suggested-by: Amit Shah <amit.shah@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Amit Shah <amit.shah@redhat.com>
Cc: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/hw/scsi/mptsas.h b/hw/scsi/mptsas.h
index da014a3..0436a33 100644
--- a/hw/scsi/mptsas.h
+++ b/hw/scsi/mptsas.h
@@ -31,6 +31,8 @@
     OnOffAuto msi;
     uint64_t sas_addr;
 
+    bool msi_in_use;
+
     /* Doorbell register */
     uint32_t state;
     uint8_t who_init;