vm-configurator: Refactor set_tablet_config()

Seperate out most of it into a more generic, set_input_config(), which
we will use to setup other input devices in the following patch.

https://bugzilla.gnome.org/show_bug.cgi?id=735768
vm-configurator: Set 'USB' bus for input devices

Lets explicitly set the bus of input devices to be USB as for some
devices libvirt/qemu will choose PS/2 by default and for keyboards (at
least), PS/2 is reported to be slower than USB.

https://bugzilla.gnome.org/show_bug.cgi?id=735768
vm-configurator: Explicitly assign mouse

This is to ensure that we have a USB mouse in new domains. While we have
not received any reports of PS/2 mouse (added by libvirt/qemu) being
slower than USB mouse, we have had reports about that about PS/2
keyboard vs. USB one. While we are adding USB keyboard, lets add USB
mouse too, just to be sure.

https://bugzilla.gnome.org/show_bug.cgi?id=735768
build: Require libvirt-gconfig >= 0.2.0

Require git master of libvirt-gconfig to be able to explicitly add
keyboard device to new domains.

https://bugzilla.gnome.org/show_bug.cgi?id=735768
vm-configurator: Explicitly assign keyboard

This is to ensure that we have a USB keyboard in new domains. We have
received reports of PS/2 keyboard (added by libvirt/qemu by default) being
slower than USB mouse (at least under Windows 8),

https://bugzilla.gnome.org/show_bug.cgi?id=735768