Create a xmlutils module to hold all the XML manipulation

Also move former xmlutils.py to xmlutils/utils.py and adjust the imports
accordingly to it.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>
Move networkxml.py to xmlutils module and update it to use lxml.builder

networkxml.py was moved to xmlutils/network.py and it was also updated
to use lxml.builder instead of plan text while generating the XML.
Update the imports accordinly to this changes and also the test cases.

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