virttest.virsh: Add secret relevant commands functions

Signed-off-by: Alex Jia <ajia@redhat.com>
virttest.libvirt_xml: Add secret XML parser module

Signed-off-by: Alex Jia <ajia@redhat.com>
libvirt.tests: Add test configuration and cases for getting and settting secret

Signed-off-by: Alex Jia <ajia@redhat.com>