Simplify the way fo getting memory usage

Linux kernel always returns same item names from /proc/meminfo.
These names are hard-coded. So we can extract the value from these
name directly, libvirt driver did this in the same way.

 * Merge small functions to one function
 * Return memory usage with total and used
 * Adjust the unit test

Change-Id: I67a7f5e5f3ac6e2bcfcd63d525806a478e2d2fb7
Merge "Simplify the way fo getting memory usage"