conf: Add interface to parse and format memory device information

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.