spapr: fix spapr-nvram migration

When spapr-nvram is backed by a file using pflash interface,
migration fails on the destination guest with assert:

    bdrv_co_pwritev: Assertion `!(bs->open_flags & 0x0800)' failed.

This avoids the problem by delaying the pflash update until after
the device loads complete.

This fix is similar to the one for the pflash_cfi01 migration:

    90c647db Fix pflash migration
Signed-off-by: NLaurent Vivier <lvivier@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/nvram/spapr_nvram.c

@@ -39,6 +39,7 @@ typedef struct sPAPRNVRAM {
     uint32_t size;
     uint8_t *buf;
     BlockBackend *blk;
+    VMChangeStateEntry *vmstate;
 } sPAPRNVRAM;
 
 #define TYPE_VIO_SPAPR_NVRAM "spapr-nvram"
@@ -185,19 +186,25 @@ static int spapr_nvram_pre_load(void *opaque)
     return 0;
 }
 
+static void postload_update_cb(void *opaque, int running, RunState state)
+{
+    sPAPRNVRAM *nvram = opaque;
+
+    /* This is called after bdrv_invalidate_cache_all.  */
+
+    qemu_del_vm_change_state_handler(nvram->vmstate);
+    nvram->vmstate = NULL;
+
+    blk_pwrite(nvram->blk, 0, nvram->buf, nvram->size, 0);
+}
+
 static int spapr_nvram_post_load(void *opaque, int version_id)
 {
     sPAPRNVRAM *nvram = VIO_SPAPR_NVRAM(opaque);
 
     if (nvram->blk) {
-        int alen = blk_pwrite(nvram->blk, 0, nvram->buf, nvram->size, 0);
-
-        if (alen < 0) {
-            return alen;
-        }
-        if (alen != nvram->size) {
-            return -1;
-        }
+        nvram->vmstate = qemu_add_vm_change_state_handler(postload_update_cb,
+                                                          nvram);
     }
 
     return 0;