conf: Add interface to parse and format memory device information
    
    This patch adds code that parses and formats configuration for memory
    devices.
    
    A simple configuration would be:
    <memory model='dimm'>
      <target>
        <size unit='KiB'>524287</size>
        <node>0</node>
      </target>
    </memory>
    
    A complete configuration of a memory device:
    <memory model='dimm'>
      <source>
        <pagesize unit='KiB'>4096</pagesize>
        <nodemask>1-3</nodemask>
      </source>
      <target>
        <size unit='KiB'>524287</size>
        <node>1</node>
      </target>
    </memory>
    
    This patch preemptively forbids use of the <memory> device in individual
    drivers so the users are warned right away that the device is not
    supported.