Setup

Remove sgio.test since this directory no longer exists.

Add support for using libiscsi resources if/when libiscsi-python module is installed.
The support is added at runtime, conditional on whether the import statement was able to load the module or not.
Future changes should probably make the sgio module load conditionally like this as well since it will allow using python-scsi on platforms that do not have an ioctl(SG_IO) interface.