sev/i386: add sev_get_capabilities()

sev/i386: add sev_get_capabilities()

The function can be used to get the current SEV capabilities.
The capabilities include platform diffie-hellman key (pdh) and certificate
chain. The key can be provided to the external entities which wants to
establish a trusted channel between SEV firmware and guest owner.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>