vmw_balloon: update maintainers list

Julien will be replacing me as the vmw_balloon maintainer.

Cc: Julien Freche <jfreche@vmware.com>
Signed-off-by: Xavier Deguillard <xdeguillard@vmware.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>