libvirt_xml-accessor: modify class XMLElementInt.

Add a carry into __slots__ and make XMLElementInt be able to
handle more kinds of integer (binary, octal, decimal and hexadecimal).

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add unittest for new XMLElementInt

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>