Using the 'uuid' element for ppc64 NVDIMM memory added in the
previous patch, use it in qemuBuildMemoryDeviceStr() to pass
it over to QEMU.

Another ppc64 restriction is the necessity of a mem->labelsize,
given than ppc64 only support label-area backed NVDIMMs.

Finally, we don't want ppc64 NVDIMMs to align up due to the
high risk of going beyond the end of file with a 256MiB
increment that the user didn't predict. Align it down
instead. If target size is less than the minimum of
256MiB + labelsize, error out since QEMU will error out
if we attempt to round it up to the minimum.
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>