tools: reimplement virt-pki-validate in C

tools: reimplement virt-pki-validate in C

The virt-pki-validate tool is currently a shell script. We have a
general goal of eliminating use of shell in the project. By doing a
new implementation in C, we can also make use of our more thorough
sanity checking code to validate the certificate setup.

This new implementation the same output format as the host validation
tool for a more consistent user experiance.

It also eliminates the requirement to have certtool installed on
libvirt hosts, which has been an issue for Fedora flatpak packages
since certtool isn't in the default platform runtime.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>