block-backend: Introduce blk_drain()

This patch introduces the blk_drain() function which allows to replace
blk_drain_all() when only one BlockDriverState needs to be drained.

Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1434537440-28236-2-git-send-email-yarygin@linux.vnet.ibm.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/block-backend.c

diff --git a/block/block-backend.c b/block/block-backend.c
index 93e46f3..aee8a12 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -700,6 +700,11 @@
     return bdrv_flush_all();
 }
 
+void blk_drain(BlockBackend *blk)
+{
+    bdrv_drain(blk->bs);
+}
+
 void blk_drain_all(void)
 {
     bdrv_drain_all();

include/sysemu/block-backend.h

diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index b4a4d5e..8fc960f 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -118,6 +118,7 @@
 int blk_co_flush(BlockBackend *blk);
 int blk_flush(BlockBackend *blk);
 int blk_flush_all(void);
+void blk_drain(BlockBackend *blk);
 void blk_drain_all(void);
 BlockdevOnError blk_get_on_error(BlockBackend *blk, bool is_read);
 BlockErrorAction blk_get_error_action(BlockBackend *blk, bool is_read,