Fix -snapshot deleting images on disk change

Block device change command did not copy BDRV_O_SNAPSHOT flag. Thus
the new image did not have this flag and the file got deleted during
opening.

Fix by copying BDRV_O_SNAPSHOT flag.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit 199630b6)
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

block.c

@@ -1811,6 +1811,11 @@ int bdrv_can_snapshot(BlockDriverState *bs)
     return 1;
 }
 
+int bdrv_is_snapshot(BlockDriverState *bs)
+{
+    return !!(bs->open_flags & BDRV_O_SNAPSHOT);
+}
+
 BlockDriverState *bdrv_snapshots(void)
 {
     BlockDriverState *bs;

block.h

@@ -202,6 +202,7 @@ const char *bdrv_get_encrypted_filename(BlockDriverState *bs);
 void bdrv_get_backing_filename(BlockDriverState *bs,
                                char *filename, int filename_size);
 int bdrv_can_snapshot(BlockDriverState *bs);
+int bdrv_is_snapshot(BlockDriverState *bs);
 BlockDriverState *bdrv_snapshots(void);
 int bdrv_snapshot_create(BlockDriverState *bs,
                          QEMUSnapshotInfo *sn_info);

blockdev.c

@@ -590,6 +590,7 @@ int do_change_block(Monitor *mon, const char *device,
         return -1;
     }
     bdrv_flags = bdrv_is_read_only(bs) ? 0 : BDRV_O_RDWR;
+    bdrv_flags |= bdrv_is_snapshot(bs) ? BDRV_O_SNAPSHOT : 0;
     if (bdrv_open(bs, filename, bdrv_flags, drv) < 0) {
         qerror_report(QERR_OPEN_FILE_FAILED, filename);
         return -1;