win32: Read serial info directly when not available through WMI

The SerialNumber field in the WMI table 'Win32_DiskDrive' is only
available from Windows Vista and Windows 2008 Server.

This patch works around this issue by querying the device directly,
but only if the SerialNumber field is not available.

Change-Id: Id39914464377c388b8ef3e2551a47a295de4a750
Bug-Url: https://bugzilla.redhat.com/1158907
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>