Introduce NVDIMM memory model

Introduce NVDIMM memory model

NVDIMM is new type of memory introduced into QEMU 2.6. The idea
is that we have a Non-Volatile memory module that keeps the data
persistent across domain reboots.

At the domain XML level, we already have some representation of
'dimm' modules. Long story short, NVDIMM will utilize the
existing <memory/> element that lives under <devices/> by adding
a new attribute 'nvdimm' to the existing @model and introduce a
new <path/> element for <source/> while reusing other fields. The
resulting XML would appear as:

    <memory model='nvdimm'>
      <source>
        <path>/tmp/nvdimm</path>
      </source>
      <target>
        <size unit='KiB'>523264</size>
        <node>0</node>
      </target>
      <address type='dimm' slot='0'/>
    </memory>

So far, this is just a XML parser/formatter extension. QEMU
driver implementation is in the next commit.

For more info on NVDIMM visit the following web page:

    http://pmem.io/

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>