virttest.virsh: Add virsh filter group functions

Add five network filter functions under filter group

Signed-off-by: Wayne Sun <gsun@redhat.com>
virttest.utils_test: Add load_python_module method

The get method in virttest.libvirt_xml.devices.librarian is
for load python module in certain path with given name, the name
is restricted in pre-defined type list. When adding network filter
rule protocol xml modules, also need to use this method to load
specific protocol xml module. So extract the get method and put it
into utils_test as load_python_module for common use.

Update get method in virttest.libvirt_xml.devices.librarian to
use the new load_python_module method, which will not broken exist
modules and tests.

Add librarian module for network filter protocols, which only have
one get method to use the load_python_module for load specific
network filter protocol xml module.

Signed-off-by: Wayne Sun <gsun@redhat.com>
virttest.libvirt_xml: Add network filter rule protocol modules under nwfilter_protocols

Add 24 libvirt nwfilter protocols modules for supported types.

This is for cover nwfilter rule protocols in page:
http://libvirt.org/formatnwfilter.html#nwfelemsRulesProto

Note:
Total number of libvirt supported protocols is 25, only igmp-ipv6
(fail for define, need confirm bug) is not included in this commit.

Signed-off-by: Wayne Sun <gsun@redhat.com>
virttest.libvirt_xml: Add nwfilter xml module

Module simplifying manipulation of XML described at
http://libvirt.org/formatnwfilter.html

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1410 from autotest/1194

PR 1194 remake