Machine provides API for restarting

Currently this is only actually implemented by LibvirtMachine as its the
only machine that needs to be restarted if user changes some of its
properties.

https://bugzilla.gnome.org/show_bug.cgi?id=710300
properties: Remove 'Reboot required' infobar

This infobar is redundant and looks ugly. We do present a notification
that says basically the same thing and it even offers a button to
actually restart the machine.

https://bugzilla.gnome.org/show_bug.cgi?id=710300
properties: Tame the 'Reboot required' notifications

Instead of annoying user with these notifications each time they make a
change, lets just show it once when user is done changing all the
properties. Notification is only shown if any of the properties changed
needs a restart to be in effect.

https://bugzilla.gnome.org/show_bug.cgi?id=710300
properties: Improved 'reboot required' message

Don't present 'Yes' and 'No' buttons as per HIG.

https://bugzilla.gnome.org/show_bug.cgi?id=710300