qemu: revert latest pSeries NVDIMM design changes

In [1], changes were made to remove the existing auto-alignment
for pSeries NVDIMM devices. That design promotes strange situations
where the NVDIMM size reported in the domain XML is different
from what QEMU is actually using. We removed the auto-alignment
and relied on standard size validation.

However, this goes against Libvirt design philosophy of not
tampering with existing guest behavior, as pointed out by Daniel
in [2]. Since we can't know for sure whether there are guests that
are relying on the auto-alignment feature to work, the changes
made in [1] are a direct violation of this rule.

This patch reverts [1] entirely, re-enabling auto-alignment for
pSeries NVDIMM as it was before. Changes will be made to ease
the limitations of this design without hurting existing
guests.

This reverts the following commits:

- commit 2d93cbdea9
  Revert "formatdomain.html.in: mention pSeries NVDIMM 'align down' mechanic"

- commit 0ee56369c8
  qemu_domain.c: change qemuDomainMemoryDeviceAlignSize() return type

- commit 07de813924
  qemu_domain.c: do not auto-align ppc64 NVDIMMs

- commit 0ccceaa57c
  qemu_validate.c: add pSeries NVDIMM size alignment validation

- commit 4fa2202d88
  qemu_domain.c: make qemuDomainGetMemorySizeAlignment() public

[1] https://www.redhat.com/archives/libvir-list/2020-July/msg02010.html
[2] https://www.redhat.com/archives/libvir-list/2020-September/msg00572.html

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>

tests/qemuxml2xmloutdata/memory-hotplug-nvdimm-ppc64.xml

@@ -38,7 +38,7 @@
         <path>/tmp/nvdimm</path>
       </source>
       <target>
-        <size unit='KiB'>786560</size>
+        <size unit='KiB'>550000</size>
         <node>0</node>
         <label>
           <size unit='KiB'>128</size>