libvirt_scsi: Fix some bugs.

1) Use `EXAMPLE*` for disk device type.
2) Raise `TestNAError` when dependent `mkisofs` don't exists.
3) Cleanup image files after test.
4) Check partition first to skip before other disk image are
   created.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #289 from Hao-Liu/fix_libvirt_scsi

libvirt_scsi: Fix some bugs.