libvirt-machine-properties: Add get_resources_properties()
libvirt-machine-properties: Add get_resources_properties()

Refactor the code that adds system resources (RAM and storage) into a
seperate method. We are declaring it as public API cause we'll be using
it from outside LibvirtMachineProperties in a following patch.

https://bugzilla.gnome.org/show_bug.cgi?id=741046

vbenes/gnome-boxes,vbenes/gnome-boxes