Add new testcase virsh_save_image_define

The testcase virsh_save_image_define make a test for
virsh subcommand: save-image-dumpxml save-image-define

Signed-off-by: Guannan Ren <gren@redhat.com>
Merge pull request #38 from Guannan-Ren/save_image_define

Add new testcase virsh_save_image_define