Test handling of non-existent x509 certs

Test handling of non-existent x509 certs

In commit f905cc998449c89339d0e2894a71d9a9e45293e5 a use of
uninitialized data was fixed based on a coverity report. It
turns out it was possible to trigger this issue by pointing
libvirt at non-existent certificate files, typically causing
a crash.

This adds a test case for that scenario. With the above
commit reverted, this new test case will crash with a SEGV.
With the fix applied, it passes, reporting a normal libvirt
error to the caller.

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