virttest.virsh: Add nodedev-{create, destroy} commands wrapper function

Signed-off-by: Alex Jia <ajia@redhat.com>
virttest.libvirt_xml.nodedev_xml: Add new elements and attribute

Signed-off-by: Alex Jia <ajia@redhat.com>
libvirt.tests: Add test cases for virsh nodedev-{create, destroy}

Signed-off-by: Alex Jia <ajia@redhat.com>
Merge pull request #983 from chuanchang/nodedev_create_destroy_test

Add test cases for creating and destroying node device