If I look at the documentation, it seems like we need to specify a compute node:

usage: nova live-migration [--block-migrate] [--disk-over-commit]
                           <server> [<host>]

Migrate running instance to a new machine.

Positional arguments:
  <server>            Name or ID of server.
  <host>              destination host name.

Optional arguments:
  --block-migrate     True in case of block_migration.
                      (Default=False:live_migration)
  --disk-over-commit  Allow overcommit.(Default=False)