conf: Introduce 'readonly' element into xml for NVDIMM memory

conf: Introduce 'readonly' element into xml for NVDIMM memory

The 'readonly' option allows users to mark vNVDIMM read-only:

<devices>
  ...
  <memory model='nvdimm' access='shared'>
      <source>
          <path>/dev/dax0.0</path>
      </source>
      <target>
          <size unit='MiB'>4094</size>
          <node>0</node>
          <label>
              <size unit='MiB'>2</size>
          </label>
          <readonly/>
      </target>
  </memory>
  ...
</devices>

Signed-off-by: Luyao Zhong <luyao.zhong@intel.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>