util: API to detect if libvirt bridge net is available

We do this by checking if:

* System libvirtd is running
* Qemu is allowed to run a helper to access bridge net.
  WARNING: We only look at a hardcoded path currently so it will fail for
  qemu installed in custom prefix.

https://bugzilla.gnome.org/show_bug.cgi?id=677688
installer: Use bridge net if system libvirt is available

Use bridge networking in new VMs, if available. This network is much
faster than the user networking.

Unfortunately bridge networking being available implies running system
libvirtd. However since system libvirtd is typically started on boot when
libvirt is installed, we can perhaps rely on it being available at
subsequent launch of the created VM in future?

NB: We don't need to actually connect to system libvirt for this to work.

https://bugzilla.gnome.org/show_bug.cgi?id=677688
libvirt-machine-props: Fix visibility of a few methods

https://bugzilla.gnome.org/show_bug.cgi?id=677688
libvirt-machine-props: Allow changing network iface

Allow changing network interface from properties view.

Co-author: Alexander Larsson <alexl@redhat.com>

https://bugzilla.gnome.org/show_bug.cgi?id=677688