block migration: Activate image on destination before writing to it

When using 'migrate -b', we must make sure to take ownership of the
image before writing to it. Otherwise metadata would be thrown away on
migration completion; this was caught by the assertions introduced in
commit 09e0c771.
Reported-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

migration/block.c

@@ -786,6 +786,7 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
     int64_t addr;
     BlockDriverState *bs, *bs_prev = NULL;
     BlockBackend *blk;
+    Error *local_err = NULL;
     uint8_t *buf;
     int64_t total_sectors = 0;
     int nr_sectors;
@@ -824,6 +825,12 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
                                  device_name);
                     return -EINVAL;
                 }
+
+                bdrv_invalidate_cache(bs, &local_err);
+                if (local_err) {
+                    error_report_err(local_err);
+                    return -EINVAL;
+                }
             }
 
             if (total_sectors - addr < BDRV_SECTORS_PER_DIRTY_CHUNK) {