migration: Fix leak of block_bitmap_mapping

migration: Fix leak of block_bitmap_mapping

Caught by inspection, but ASAN also reports:

Direct leak of 16 byte(s) in 1 object(s) allocated from:
 #0 in malloc
 #1 in g_malloc
 #2 in g_memdup
 #3 in qapi_clone_start_struct ../qapi/qapi-clone-visitor.c:40:12
 #4 in qapi_clone_start_list ../qapi/qapi-clone-visitor.c:59:12
 #5 in visit_start_list ../qapi/qapi-visit-core.c:80:10
 #6 in visit_type_BitmapMigrationNodeAliasList qapi/qapi-visit-migration.c:639:10
 #7 in migrate_params_apply ../migration/options.c:1407:13
 #8 in qmp_migrate_set_parameters ../migration/options.c:1463:5
 #9 in qmp_marshal_migrate_set_parameters qapi/qapi-commands-migration.c:214:5
 #10 in do_qmp_dispatch_bh ../qapi/qmp-dispatch.c:128:5

Note that this is entirely harmless because the migration object which
contains the MigrationParameters structure is kept until the QEMU
process exits.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20251215220041.12657-2-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>