block: Add blk_remove_bs()

This function removes the BlockDriverState associated with the given
BlockBackend from that BB and sets the BDS pointer in the BB to NULL.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/block-backend.c

diff --git a/block/block-backend.c b/block/block-backend.c
index 19fdaae..878c448 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -334,6 +334,18 @@
 }
 
 /*
+ * Disassociates the currently associated BlockDriverState from @blk.
+ */
+void blk_remove_bs(BlockBackend *blk)
+{
+    blk_update_root_state(blk);
+
+    blk->bs->blk = NULL;
+    bdrv_unref(blk->bs);
+    blk->bs = NULL;
+}
+
+/*
  * Associates a new BlockDriverState with @blk.
  */
 void blk_insert_bs(BlockBackend *blk, BlockDriverState *bs)

include/sysemu/block-backend.h

diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 9306a52..14a6d32 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -72,6 +72,7 @@
 BlockBackend *blk_next(BlockBackend *blk);
 
 BlockDriverState *blk_bs(BlockBackend *blk);
+void blk_remove_bs(BlockBackend *blk);
 void blk_insert_bs(BlockBackend *blk, BlockDriverState *bs);
 
 void blk_hide_on_behalf_of_hmp_drive_del(BlockBackend *blk);