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).
From comment #5, i was able to make changes persistent with access='shared':
<memory model='nvdimm' access='shared'>
<path> /tmp/.nbdpath1. 2898617< /path> >1048576< /size>
<node> 0</node> >256</size>
<source>
</source>
<target>
<size unit='KiB'
<label>
<size unit='KiB'
</label>
</target>
<address type='dimm' slot='0'/>
</memory>
so now backing files are updated (and can even be shared among 2 vms).