virttest.libvirt_xml: Unify value of default param virsh_instance

Fix the inconsistencies in some virsh_instance default
param assignments. This will make it easier to move libvirt_xml
to tp-libvirt.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.libvirt_xml: Move load_python_module to base

The deceptively generic function name load_python_module
hid the fact that this was an API with an application
way too narrow to be in virttest.utils_test. So move it
back to the virttest.libvirt_xml module where it belongs.

Also, change the API name slightly to load_xml_module.
This is not a generic python module loader function,
it has a very specific application.

CC: Wayne Sun <gsun@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1428 from autotest/untangle-deps

Untangle deps