The output for aarch64[1] `lshw -quiet -json` is a bit different from
x86_64, this will lead to 0 physical memory calculated currently. This
patch aims to make it compatible for aarch64 when getting physical memory
[1]: http://paste.openstack.org/show/754789/
Change-Id: Ib7b4ee857d8550e3d75ac8a53c61f0ecab45feff