Implement bdrv_co_block_status for snapshot export support

docs/usage/cli.en.md

@@ -14,6 +14,7 @@ It supports the following commands:
 - [df](#df)
 - [ls](#ls)
 - [create](#create)
+- [snap-create](#create)
 - [modify](#modify)
 - [rm](#rm)
 - [flatten](#flatten)
@@ -123,6 +124,8 @@ vitastor-cli snap-create [-p|--pool <id|name>] <image>@<snapshot>
 
 Create a snapshot of image `<name>` (either form can be used). May be used live if only a single writer is active.
 
+See also about [how to export snapshots](qemu.en.md#exporting-snapshots).
+
 ## modify
 
 `vitastor-cli modify <name> [--rename <new-name>] [--resize <size>] [--readonly | --readwrite] [-f|--force]`