CP-8679: fake makedirs and file, add errorcodes

Enhance the testing library by faking out os.makedirs and file. This
will enable to test further components, which try to use these
functions.

Also create a setup_error_codes method that will pretend that the error
codes file required by XenError are there.

Also increase the existing testlib's coverage to 100%.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
Fix XenError

If the XML error description does not exist in the filesystem, XenError
tried to call Exception.__init__(self, ''), which is invalid, as self is
not inheriting from Exception.

Github: fixes xapi-project/sm#158

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>