domcaps: Report launchSecurity

domcaps: Report launchSecurity

In order to learn what types of <launchSecurity/> are supported
users can turn to domain capabilities and find <sev/> and
<s390-pv/> elements. While these may expose some additional info
on individual launchSecurity types, we are lacking clean
enumeration (like we do for say device models). And given that
SEV and SEV SNP share the same basis (info found under <sev/> is
applicable to SEV SNP too) we have no other way to report SEV SNP
support.

Therefore, report supported launchSecurity types in domain
capabilities.

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