hv_xen: Factorize and test disk configuration

The “_GetConfigFileDiskData” function is moved to module level and
cleaned up (module-level constants for letters and file I/O drivers).

Until now only 24 disks would be supported (e.g. “sda” to “sdx”), when
the Latin alphabet actually has 26 characters. Now all 26 letters would
be available for use (“constants.MAX_DISKS” is still 16).

Newly added unit tests provide complete coverage for
“_GetConfigFileDiskData”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>