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.

On RHEL 5, I noticed this test failure message:

TEST: qemumonitorjsontest
libvirt not compiled with yajl, skippingSKIP: qemumonitorjsontest

* tests/virstoragetest.c (testPrepImages): Use simpler fputs.
* tests/qemumonitorjsontest.c (mymain): Ensure trailing newline.

Testing our backing chain handling will make it much easier to
ensure that we avoid issues in the future.  If only I had written
this test before I first caused several regressions...

* tests/virstoragetest.c: New test.
* tests/Makefile.am (test_programs): Build it.
* .gitignore: Ignore new files.