If the 'service' property is not set, we'll call pam_start() with a NULL
pointer for the service name. This fails and leaves a message like this
in the syslog:

qemu-storage-daemon[294015]: PAM pam_start: invalid argument: service == NULL

Make specifying the property mandatory and catch the error already
during the creation of the object.

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