block: copy enable_write_cache in bdrv_append

Because the guest will be able to flip enable_write_cache, the actual
state may not match what is used to open the new snapshot.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

diff --git a/block.c b/block.c
index 7538112..9bff401 100644
--- a/block.c
+++ b/block.c
@@ -1000,6 +1000,8 @@
     tmp.buffer_alignment  = bs_top->buffer_alignment;
     tmp.copy_on_read      = bs_top->copy_on_read;
 
+    tmp.enable_write_cache = bs_top->enable_write_cache;
+
     /* i/o timing parameters */
     tmp.slice_time        = bs_top->slice_time;
     tmp.slice_start       = bs_top->slice_start;