qemuimage-testlib: Add check for existence of image and correctness of ip address
Fixes [BUGID #612, #611]
Add check for existence of image to be tested in qemuimage-testlib.
This ensures that sanity test returns failure immediatly when there is
no image found. And also add check for the correctness of ip address.
If the ip address returned by function Test_Fetch_Target_IP is 0, it
means qemu starts up failed and no valid ip address found.
