conf: Introduce optional 'uuid' element for NVDIMM memory

conf: Introduce optional 'uuid' element for NVDIMM memory

ppc64 NVDIMM support was implemented in QEMU by commit [1].
The support is similar to what x86 already does, aside from
an extra 'uuid' element.

This patch introduces a new optional 'uuid' element for the
NVDIMM memory model. This element behaves like the 'uuid'
element of the domain definition - if absent, we'll create
a new one, otherwise use the one provided by the XML.
The 'uuid' element is exclusive to pseries guests and are
unavailable for other architectures.

Next patch will use this new element to add NVDIMM support
for ppc64.

[1] https://github.com/qemu/qemu/commit/ee3a71e36654317b14ede0290e87628f8b79f850

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