audit: properly encode device path in cgroup audit

audit: properly encode device path in cgroup audit

https://bugzilla.redhat.com/show_bug.cgi?id=922186

Commit d04916fa introduced a regression in audit quality - even
though the code was computing the proper escaped name for a
path, it wasn't feeding that escaped name on to the audit message.
As a result, /var/log/audit/audit.log would mention a pair of
fields class=path path=/dev/hpet instead of the intended
class=path path="/dev/hpet", which in turn caused ausearch to
format the audit log with path=(null).

* src/conf/domain_audit.c (virDomainAuditCgroupPath): Use
constructed encoding.

Signed-off-by: Eric Blake <eblake@redhat.com>