Don't record RRDs for the "ovs-system" interface

This started appearing in the recent single-datapath OVSes, but does not really
add anything useful.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #1830 from robhoes/ovs-system

Don't record RRDs for the "ovs-system" interface
CA-140545: wrapper should exit after launching vncterm

To avoid an instance of fe hanging around for each vncterm we launch,
fork vncterm and exit rather than calling exec.

This means that the script has to write the child pid ($!) rather than
its own pid ($$) to xenstore.

Signed-off-by: John Else <john.else@citrix.com>