conf: Introduce virtio-pmem <memory/> model

conf: Introduce virtio-pmem <memory/> model

The virtio-pmem is a virtio variant of NVDIMM and just like
NVDIMM virtio-pmem also allows accessing host pages bypassing
guest page cache. The difference is that if a regular file is
used to back guest's NVDIMM (model='nvdimm') the persistence of
guest writes might not be guaranteed while with virtio-pmem it
is.

To express this new model at domain XML level, I've chosen the
following:

  <memory model='virtio-pmem' access='shared'>
    <source>
      <path>/tmp/virtio_pmem</path>
    </source>
    <target>
      <size unit='KiB'>524288</size>
    </target>
    <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
  </memory>

Another difference between NVDIMM and virtio-pmem is that while
the former supports NUMA node locality the latter doesn't. And
also, the latter goes onto PCI bus and not into a DIMM module.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>