libvirt-broker: Make add_domain async

... and in turn, some of the other private functions in LibvirtBroker.
This will later be necessary when we adjust the domain's cpu mode when
adding it.

https://bugzilla.gnome.org/show_bug.cgi?id=735685
app: Add constant for default source name

https://bugzilla.gnome.org/show_bug.cgi?id=735685
vm-configurator: Add is_boxes_installed

is_boxes_installed determines if the given domain is done installing.

https://bugzilla.gnome.org/show_bug.cgi?id=735685
libvirt-machine-props: No snapshots during installations

We don't want to allow taking snapshots during an installation,
so just don't show the snapshots page in the sidebar if an installation
is in progress.

https://bugzilla.gnome.org/show_bug.cgi?id=735685
LibvirtBroker: Add & use update_existing_domain

This function will be responsible for updating configuration of
existing/old domains that were created by Boxes. For now it only updates
the "host-passthrough" CPU model using configurations to use custom CPU
model instead (just like we do for new domains now).

LibvirtBroker calls this function on all domains on the default
(session) libvirt connection.

This enables snapshots for existing/old domains that are still using
"host-passthrough".

https://bugzilla.gnome.org/show_bug.cgi?id=735685