Add a test case for the fdstream file read/write code

Add a test case for the fdstream file read/write code

Add a test case which exercises the virFDStreamOpenFile
and virFDStreamCreateFile methods. Ensure that both the
synchronous and non-blocking iohelper code paths work.
This validates the regression recently fixed which
broke reading in non-blocking mode

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>