migration: Postpone releasing MigrationState.hostname

migration: Postpone releasing MigrationState.hostname

We used to release it right after migrate_fd_connect().  That's not good
enough when there're more than one socket pair required, because it'll be
needed to establish TLS connection for the rest channels.

One example is multifd, where we copied over the hostname for each channel
but that's actually not needed.

Keeping the hostname until the cleanup phase of migration.

Cc: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220331150857.74406-2-peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Fixup checkpatch error; don't need to check for NULL
  around g_free