build: Require libvirt-gconfig >= 0.1.9

We'll need the latest version to make use of new CPU model API.

https://bugzilla.gnome.org/show_bug.cgi?id=732680
vm-configurator: Explicitly assign host cpu model

Instead of using 'host-passthrough' mode, use custom mode and set the
model of CPU explicitly to that of the host CPU. While 'host-model' mode
still has issues and should be avoided, turns out that 'host-passthrough'
does not work for non-KVM.

This patch follows the advice given in libvirt documentation.

From http://libvirt.org/formatdomain.html#elementsCPU

"Beware, due to the way libvirt detects host CPU and due to the fact
libvirt does not talk to QEMU/KVM when creating the CPU model, CPU
configuration created using host-model may not work as expected. The
guest CPU may differ from the configuration and it may also confuse
guest OS by using a combination of CPU features and other parameters
(such as CPUID level) that don't work. Until these issues are fixed,
it's a good idea to avoid using host-model and use custom mode with just
the CPU model from host capabilities XML."

https://bugzilla.gnome.org/show_bug.cgi?id=732680