libvirt-machine-props,ovirt-machine: Drop weird term 'Virtualizer'

Replace use of weird term 'Virtualizer' in the UI with a more
friendlier (and used elsewhere in the UI) term, 'Broker'.

https://bugzilla.gnome.org/show_bug.cgi?id=710294
libvirt-machine-props,ovirt-machine: Show broker name

Instead of showing broker's URL (that won't mean much to user), show the
name of the broker.

https://bugzilla.gnome.org/show_bug.cgi?id=710294
libvirt-machine-props: Broker URI in troubleshooting log

https://bugzilla.gnome.org/show_bug.cgi?id=710294