libvirt.tests: Add a test for disk startupPolicy testing

libvirt domain disk XML may has a startupPolicy element:
    <disk type='file' device='floppy'>
      <source file='xxx' startupPolicy='mandatory'/>
      <target dev='fda' bus='fdc'/>
    </disk>
The value may be mandatory, requisite or optional, which corresponds to
differet behavior when start domain. This test case aim to test the three
different values for CD-ROM and floppy disks.

Signed-off-by: Yanbing Du <ydu@redhat.com>