Update license to LGPLv3

As Kimchi depends on some imported code (src/kimchi/websocket.py and
src/kimchi/websockify.py) under LGPLv3, its license also needs to be upgraded
to LGPLv3.

COPYING.LGPL was updated according to http://www.gnu.org/licenses/lgpl-3.0.txt

As all Kimchi files have license header indicating LGPL 2.1 or later,
they don't need to be updated.

For reference: http://gplv3.fsf.org/dd3-faq

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>