block: support passing 'backing': '' to 'blockdev-add'

Passing an empty string allows opening an image but not its backing
file. This was already described in the API documentation, only the
implementation was missing.

This is useful for creating snapshots using images opened with
blockdev-add, since they are not supposed to have a backing image
before the operation.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

diff --git a/block.c b/block.c
index a99e6d8..3493501 100644
--- a/block.c
+++ b/block.c
@@ -1393,6 +1393,7 @@
     BlockDriverState *bs;
     BlockDriver *drv = NULL;
     const char *drvname;
+    const char *backing;
     Error *local_err = NULL;
     int snapshot_flags = 0;
 
@@ -1460,6 +1461,12 @@
 
     assert(drvname || !(flags & BDRV_O_PROTOCOL));
 
+    backing = qdict_get_try_str(options, "backing");
+    if (backing && *backing == '\0') {
+        flags |= BDRV_O_NO_BACKING;
+        qdict_del(options, "backing");
+    }
+
     bs->open_flags = flags;
     bs->options = options;
     options = qdict_clone_shallow(options);