virttest.libvirt_xml: Add attributes to Video class

Add attributes to Video class to support video device operation.

Signed-off-by: Wayne Sun <gsun@redhat.com>
virttest.virsh: Add screeshot_test function

The screenshot function is used by screendump in VM class, which
lack screen option and will raise error and fail test in negative
test.

Add the screenshot_test function which return CmdResult for test
the screenshot command in libvirt test provider.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1406 from waynesun09/virsh_screenshot

virttest: Add support for video device operation and screenshot_test function