vm-creator: Refactor config creation code

Put config creation code into a separate method: create_domain_config.
We keep it protected and virtual cause in a later patch we'll override
this from a new subclass.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
vm-creator: Refactor create_domain_name_and_title_from_media

Seperate out creation of base name and title to seperate virtual
protected method so subclasses can provide their own implementation for
this.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
installed-media: Rename 'convert_to_native_format'

Rename 'convert_to_native_format' method to just 'copy'. The conversion
part is internal detail.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
installed-media: Provide a sync constructor as well

This is mainly needed as workaround for bug#712186 when we subclass
InstalledMedia in a following patch in this series.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
vm-importer: Allow subclasses to not start VM after import

https://bugzilla.gnome.org/show_bug.cgi?id=666185
wizard-source: Add 'activate' signal

Instead of users of this class listening to 'activate' signal on the URL
entry, they can now listen to 'activate' signal on wizard source itself.

This will allow us later to have a source w/o any URLs.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
wizard-source: Only show 5 medias at the same time

Five is already good enough number to show at a time and we'll need some
space for a new item in the main wizard source menu soon.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
Introducing libvirt system importing classes

a. LibvirtSystemMedia: Represents main disks of system libvirt domains.
b. LibvirtSystemVMImporter: A subclass of VMImporter that will be
   responsible for importing a VM from system libvirt.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
Allow user to import system libvirt VMs

If there is no boxes defined and there is VMs available on system
libvirt, allow user to import those VMs to session libvirt (and therefore
Boxes).

Issues/TODO:

* We gotta elevate privileges to change permissions on disks/images as by
  default they are not even readable to anyone but root/qemu. This
  unfortunately means launching of undesirable polkit dialog.

* The wizard summary in this case could use improvement/more info.

* We don't give any option to user for deleting the source VMs from
  system libvirt after import. If we do that, we'll have to trigger yet
  more polkit dialogs. :( Had a bit of discussion with jimmac about this.
  He suggested that perhaps we should track the imported VMs so that we
  only present option to import if there is something new to import.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
vm-configurator: Add setup_custom_xml()

Add a public method to setup custom XML on new domains. We'll use this
for domains imported from system libvirt, where most of the config is
mostly copied from an existing domain. Therefore create_domain_config()
is not used and that means update_custom_xml is never called either.

https://bugzilla.gnome.org/show_bug.cgi?id=666185
Restore system libvirt imports from previous runs

Save the state for system libvirt imports so that if Boxes is quite
before import is finished, the import is restarted next time Boxes is
run.

https://bugzilla.gnome.org/show_bug.cgi?id=666185