Increase test coverage

Increases the test coverage on many of the XML handling functions and code around them. Had to do a few fixes in Guest.py and TDL.py to make it easier to test and more consistent with other XML passing around.

Also tested `distrolist()` in GuestFactory. It does mean when any new versions of a distro comes out that tests has to be updated but that can also help us.

I wasn't too sure if the additional should live in the TDL test file given how most things are being tested in there but I ended up doing what test_peristant was done like.

Coverage before:
```
Name              Stmts   Miss  Cover
-------------------------------------
oz/Debian            54     34    37%
oz/Fedora            43     18    58%
oz/FedoraCore        25      7    72%
oz/Guest           1071    820    23%
oz/GuestFactory      23      9    61%
oz/Mageia            50     50     0%
oz/Mandrake          77     50    35%
oz/Mandriva          50     31    38%
oz/OpenSUSE         282    232    18%
oz/OzException        4      0   100%
oz/RHEL_2_1          15      1    93%
oz/RHEL_3            39     18    54%
oz/RHEL_4            47     24    49%
oz/RHEL_5            53     30    43%
oz/RHEL_6            26      7    73%
oz/RHL               44     14    68%
oz/RedHat           659    569    14%
oz/TDL              239     41    83%
oz/Ubuntu           440    353    20%
oz/Windows           94     54    43%
oz/__init__           0      0   100%
oz/linuxutil         11      8    27%
oz/ozutil           468    144    69%
-------------------------------------
TOTAL              3814   2514    34%
```

After:
```
Name              Stmts   Miss  Cover
-------------------------------------
oz/Debian            54     33    39%
oz/Fedora            43     17    60%
oz/FedoraCore        25      6    76%
oz/Guest           1075    669    38%
oz/GuestFactory      23      0   100%
oz/Mageia            50     36    28%
oz/Mandrake          77     49    36%
oz/Mandriva          50     30    40%
oz/OpenSUSE         282    231    18%
oz/OzException        4      0   100%
oz/RHEL_2_1          15      0   100%
oz/RHEL_3            39     17    56%
oz/RHEL_4            47     23    51%
oz/RHEL_5            53     29    45%
oz/RHEL_6            26      6    77%
oz/RHL               44     13    70%
oz/RedHat           659    569    14%
oz/TDL              236     16    93%
oz/Ubuntu           440    352    20%
oz/Windows           94     53    44%
oz/__init__           0      0   100%
oz/linuxutil         11      8    27%
oz/ozutil           468    144    69%
-------------------------------------
TOTAL              3815   2301    40%
```


May not seem like a lot but `Guest` and `TDL` both got a good hike

Here's how to generate the coverage reports without using junit

```
pip install coverage
py.test --genscript=runtests.py
coverage run --source oz runtests.py -vv tests
```

I had to use runtests.py to get any real coverage. It seems the way the test functionality is setup in setup.py wasn't aggreeing with coverage.py. http://pytest.org/latest/goodpractises.html uses runtests.py as well for distutils