Don't try to add non-existant devices to ACL

Don't try to add non-existant devices to ACL

The QEMU driver has a list of devices nodes that are whitelisted
for all guests. The kernel has recently started returning an
error if you try to whitelist a device which does not exist.
This causes a warning in libvirt logs and an audit error for
any missing devices. eg

2013-02-27 16:08:26.515+0000: 29625: warning : virDomainAuditCgroup:451 : success=no virt=kvm resrc=cgroup reason=allow vm="vm031714" uuid=9d8f1de0-44f4-a0b1-7d50-e41ee6cd897b cgroup="/sys/fs/cgroup/devices/libvirt/qemu/vm031714/" class=path path=/dev/kqemu rdev=? acl=rw

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