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>
virt: Add exception catching style update

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Fixup secret_set_get

* Update try...except...finally.. syntax to old-style.
* Conformed parameter keys to match rest of test
* Fix logging parameter format bug

Signed-off-by: Chris Evich <cevich@redhat.com>