docs/manpages: add checklist of problems for SEV attestation

docs/manpages: add checklist of problems for SEV attestation

Despite efforts to make the virt-qemu-sev-validate tool friendly, it is
a certainty that almost everyone who tries it will hit false negative
results, getting a failure despite the VM being trustworthy.

Diagnosing these problems is no easy matter, especially for those not
familiar with SEV/SEV-ES in general. This extra docs text attempts to
set out a checklist of items to look at to identify what went wrong.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>