block: Add errp to b{lk,drv}_truncate()

For one thing, this allows us to drop the error message generation from
qemu-img.c and blockdev.c and instead have it unified in
bdrv_truncate().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170328205129.15138-3-mreitz@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/qemu-io-cmds.c b/qemu-io-cmds.c
index 312fc6d..21af9e6 100644
--- a/qemu-io-cmds.c
+++ b/qemu-io-cmds.c
@@ -1567,6 +1567,7 @@
 
 static int truncate_f(BlockBackend *blk, int argc, char **argv)
 {
+    Error *local_err = NULL;
     int64_t offset;
     int ret;
 
@@ -1576,9 +1577,9 @@
         return 0;
     }
 
-    ret = blk_truncate(blk, offset);
+    ret = blk_truncate(blk, offset, &local_err);
     if (ret < 0) {
-        printf("truncate: %s\n", strerror(-ret));
+        error_report_err(local_err);
         return 0;
     }