Libvirt_xml module to package + Major Read/Codeability enhancements

(From git log...)
    virt-libvirt: Fixup tests for updated libvirt_xml
    virt-libvirt: Reapply dropped libvirt_xml patches

    virt-libvirt: Update libvirt_xml_unittest
    * Added new testcase class to test the accessors module
    * Updated all classes to use package/submodules
    * Updated all classes to use accessors module fully

    virt-libvirt: Package libvirt_xml w/ accessor generator
    The module was growing too rapidly to be contained within a single
    module and remain comprehensible.  Also The vast majority of
    libvirt's XML can be operated on via a small collection of
    common operations.  For example adding/removing a single tag
    somewhere in the tree.
    * Break libvirt_xml into a package, with submodules containing
      updated versions of the original classes.
    * Impliment an accessor submodule to consolidates common
      operations into generator classes of callable subclass instances.
    * The LibvirtXMLBase base-class is utilized by many subclasses
      soas to avoid circular dependencies, it was moved into its
      own 'base' module.
    * All exception definitions would also have caused circular
      dependencies, so they were moved to a dedicated xcepts module.
    * The LibvirtXML class (for working with 'capabilities') was
      moved into a  sub-module and updated to use the accessors module.
    * The network_xml module now fully support all basic aspects of
      libvirt's host-network XML format.  It was updated extensivly
      to make use of common accessor methods.  New classes were added
      to help handle IP addresses and associated DHCP ranges for XML bridge
      description.
    * The VMXML class was moved to the vm_xml module where it remains
      fairly basic/unchanged for now.  Becides converting classes to
      use the new accessor methods, little else was changed.

    virt: Small propcan module tweaks
    * Updated docs
    * Separated out Semi-private methods from PropcanBase class
      to reduce pylint noise coming from other modules.
    * Changed __setitem__ to disallow setting properties not defined in
      __slots__ (as was original intention of class)

    virt: Add method to rebase an XML element to root
    This is a convenience when you want some behavior to
    only operate on a subtree of an XML document as if
    it were a separate XML tree.

    This functionality is provided by a 'private' name
    in the ElementTree module to protect users from
    some negative implications.  This method masks
    those negative aspects and provides a public name.