virt-libvirt: De-duplicate virsh/libvirt_xml bases

The virt networking code in utils_misc and base classes from the virsh
and libvirt_xml modules were duplicating a lot of functionality.  Moved
the comonalities under a new utils_misc PropCanBase base class. Added
new unittests for the new base class and fixed up all usage mismatches.

Also moved Closure class from virsh into utils_misc so it's more
centrally located for common use.
Signed-off-by: NChris Evich <cevich@redhat.com>