move sev firmware setup to separate function so it can be used from
other code paths.  No functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20220425135051.551037-3-kraxel@redhat.com>