Use lxml.etree on xmlutils/utils.py instead of xml.etree and libxml2

xml.etree does not work well when the XML has non-registered namespaces.
And as libvirt allows anyone to insert custom XML on <metadata> element
it can be an easily point of failure while manipulating guest XML.
To avoid problems with that use lxml.etree

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Use lxml.etree on gen-index.py script instead of libxml2

That way we remove all the occurrences of libxml2 on Kimchi code and
then we don't need to depend on libxml2-python package anymore.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Remove libxml2-python as Kimchi dependency

Kimchi is now always using python-lxml for XML manipulation.
So there is not need to depend on libxml2-python

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