That's an old way to enable backend-transfer feature for
vhost-user-blk.

I just kept it unchanged since v1 of the series.

At least, it should be migration-parameter instead of
capability.

But currently, there is discussion on final API in
context of "[PATCH v8 00/19] virtio-net: live-TAP local migration", here:
https://lore.kernel.org/qemu-devel/29aa1d66-9fa7-4e44-b0e3-2ca26e77accf@yandex-team.ru/

So, consider this one as temporary patch, which will be replaced
by some other API.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 migration/options.c |  7 +++++++
 migration/options.h |  1 +
 qapi/migration.json | 11 +++++++++--
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/migration/options.c b/migration/options.c
index 5183112775..3a013b9898 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -262,6 +262,13 @@ bool migrate_mapped_ram(void)
     return s->capabilities[MIGRATION_CAPABILITY_MAPPED_RAM];
 }
 
+bool migrate_local_vhost_user_blk(void)
+{
+    MigrationState *s = migrate_get_current();
+
+    return s->capabilities[MIGRATION_CAPABILITY_LOCAL_VHOST_USER_BLK];
+}
+
 bool migrate_ignore_shared(void)
 {
     MigrationState *s = migrate_get_current();
diff --git a/migration/options.h b/migration/options.h
index 82d839709e..95a3f482bc 100644
--- a/migration/options.h
+++ b/migration/options.h
@@ -30,6 +30,7 @@ bool migrate_colo(void);
 bool migrate_dirty_bitmaps(void);
 bool migrate_events(void);
 bool migrate_mapped_ram(void);
+bool migrate_local_vhost_user_blk(void);
 bool migrate_ignore_shared(void);
 bool migrate_late_block_activate(void);
 bool migrate_multifd(void);
diff --git a/qapi/migration.json b/qapi/migration.json
index be0f3fcc12..d29a4915b4 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -517,9 +517,15 @@
 #     each RAM page.  Requires a migration URI that supports seeking,
 #     such as a file.  (since 9.0)
 #
+# @local-vhost-user-blk: Migrate vhost-user-blk locally, keeping
+#     backend alive. Open file descriptors and backend-related state
+#     are migrated. Only may be used when migration channel is UNIX
+#     domain socket. (since 10.2)
+#
 # Features:
 #
-# @unstable: Members @x-colo and @x-ignore-shared are experimental.
+# @unstable: Members @x-colo, @x-ignore-shared,
+#     @local-vhost-user-blk are experimental.
 # @deprecated: Member @zero-blocks is deprecated as being part of
 #     block migration which was already removed.
 #
@@ -536,7 +542,8 @@
            { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
            'validate-uuid', 'background-snapshot',
            'zero-copy-send', 'postcopy-preempt', 'switchover-ack',
-           'dirty-limit', 'mapped-ram'] }
+           'dirty-limit', 'mapped-ram',
+           { 'name': 'local-vhost-user-blk', 'features': [ 'unstable' ] } ] }
 
 ##
 # @MigrationCapabilityStatus:
-- 
2.48.1