machine: Make 'can-save' a virtual property

Instead of subclasses overriding the value of 'can-save' property,
let them override the property getter itself so they can implement
smarter (more dynamic) getters for it.

https://bugzilla.gnome.org/show_bug.cgi?id=730793
libvirt-machine: Smarter 'can-save' implementation

This property shoudl check the state of the machine rather than its user
having to do that in addition to checking the value of this property.

https://bugzilla.gnome.org/show_bug.cgi?id=730793
libvirt-machine: Add private 'saving' boolean field

This field is set to true if machine is being saved currently.

https://bugzilla.gnome.org/show_bug.cgi?id=730793
libvirt-machine: Not savable when being saved already

Don't claim the machine to be savable if its being saved.

This solves the issue of pause button still being available to user and
Boxes leading into interesting scenerios if user clicks on the button.

https://bugzilla.gnome.org/show_bug.cgi?id=730793