libvirt-machine: Delay starting if being saved

Connecting to a machine that is currently being saved results in an
error so lets wait for saving to finish completely before we launch the
machine.

https://bugzilla.gnome.org/show_bug.cgi?id=694931
machine: Add schedule_autosave()

Add a method to autosave the machine after 1 minute.

https://bugzilla.gnome.org/show_bug.cgi?id=694931
machine: Add unschedule_autosave()

Add a method to unschedule a previously scheduled autosave.

https://bugzilla.gnome.org/show_bug.cgi?id=694931
app-window: Autosave machines when in background

When a machine is no longer running in foreground in any window,
schedule an autosave for it.

https://bugzilla.gnome.org/show_bug.cgi?id=694931
vm-creator: Add 'expess_install' property

Add a convenient property to find out if vm-creator is doing (or going
to do) an express installation.

https://bugzilla.gnome.org/show_bug.cgi?id=694931
machine: Add protected 'should_save' property

Allow subclasses to easily override the decision to autosave or not.

https://bugzilla.gnome.org/show_bug.cgi?id=694931
libvirt-machine: Don't autosave express installs

Express installations are supposed to run in the background since the
whole point of it is for user to not interact with the box until
installation is complete.

https://bugzilla.gnome.org/show_bug.cgi?id=694931
vm-creator: Autosave VM after express install

While it doesn't make sense to autosave the VM while express
installation is going on in it, we should autosave it once express
installation is finished.

https://bugzilla.gnome.org/show_bug.cgi?id=694931