conf: Expose SEV feature in domain capabilities

conf: Expose SEV feature in domain capabilities

Extend hypervisor capabilities to include sev feature. When available,
hypervisor supports launching an encrypted VM on AMD platform. The
sev feature tag provides additional details like Platform Diffie-Hellman
(PDH) key and certificate chain which can be used by the guest owner to
establish a cryptographic session with the SEV firmware to negotiate
keys used for attestation or to provide secret during launch.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>