Comment 7 for bug 1855177

From comment #5, i was able to make changes persistent with access='shared':

    <memory model='nvdimm' access='shared'>
      <source>
        <path>/tmp/.nbdpath1.2898617</path>
      </source>
      <target>
        <size unit='KiB'>1048576</size>
        <node>0</node>
        <label>
          <size unit='KiB'>256</size>
        </label>
      </target>
      <address type='dimm' slot='0'/>
    </memory>

so now backing files are updated (and can even be shared among 2 vms).