crypto/linux_keyring: add 'secret_keyring' secret object.

crypto/linux_keyring: add 'secret_keyring' secret object.

Add the ability for the secret object to obtain secret data from the
Linux in-kernel key managment and retention facility, as an extra option
to the existing ones: reading from a file or passing directly as a
string.

The secret is identified by the key serial number. The upper layers
need to instantiate the key and make sure the QEMU process has access
permissions to read it.

Signed-off-by: Alexey Krasikov <alex-krasikov@yandex-team.ru>
 - Fixed up detection logic default behaviour in configure

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>