libvirt plugin: Fix the HostnameFormat option.

libvirt plugin: Fix the HostnameFormat option.

The attached patch fixes HostnameFormat in libvirt driver.

The intended use for this was:

   HostnameFormat "name"

generating hostnames like "<guestname>" (the default case), and:

   HostnameFormat "hostname name"

generating hostnames like "<hostname>:<guestname>".

Somewhere along the way, however, that code got broken so it always adds a ":"
before the name, and doesn't concatenate any more, so you'd get weird hostnames
appearing like ":guest" which was supposed to be "hostname:guest".

The patch just reverts back to my original code.

Rich.