secrets: add support for running secret driver in embedded mode

secrets: add support for running secret driver in embedded mode

This enables support for running the secret driver embedded to the
calling application process using a URI:

   secret:///embed?root=/some/path

When using the embedded mode with a root=/var/tmp/embed, the
driver will use the following paths:

       configDir: /var/tmp/embed/etc/secrets
        stateDir: /var/tmp/embed/run/secrets

These are identical whether the embedded driver is privileged
or unprivileged.

This compares with the system instance which uses

       configDir: /etc/libvirt/secrets
        stateDir: /var/lib/libvirt/secrets

When an embedded instance of the secret driver is open, any other
embedded drivers will automatically use the embedded secret driver.

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