Add change-media function in virsh module

Signed-off-by: whuang <whuang@redhat.com>
libvirt: add change-media testing

Test command: virsh change-media.

The command changes the media used by CD or floppy drives.

Test steps:
1. Prepare test environment.
2. Perform virsh change-media operation.
3. Recover test environment.
4. Confirm the test result.

Changes from v1:

 * Replaced needless use of shell commands
   when there is a valid python API that can
   be used instead
 * Fixed check of files inside a cdrom device
   inside the guest
 * Normalized references to CDrom to cdrom.
 * Marked tests requiring libvirt restart to
   require_root
 * Squashed test and config on a same commit

Signed-off-by: whuang <whuang@redhat.com>