tests: skip virstoragetest on RHEL 5

tests: skip virstoragetest on RHEL 5

virstoragetest was failing on RHEL 5, but with no good error message:

TEST: virstoragetest
                                        0   FAIL

It turns out that qemu-img was so old, that it lacked support for
-o backing_file.  It didn't help that the test was also using
qemu-img from PATH, even after first probing for kvm-img.

* tests/virstoragetest.c (testPrepImages): Consistently use
discovered binary.  Skip instead of fail if qemu-img fails during
setup.