libvirt: speed up get_memory_mb_used method

Switch the get_memory_mb_used method to use the new
list_instance_domains method.

Blueprint: libvirt-domain-listing-speedup
Change-Id: I6f9e9a2ee761b81e8fc06237cfef1b7202effe37
Merge "libvirt: speed up get_memory_mb_used method"