qemu: Don't prealloc mem for real NVDIMMs

qemu: Don't prealloc mem for real NVDIMMs

Currently, we configure QEMU to prealloc memory almost by
default. Well, by default for NVDIMMs, hugepages and if user
asked us to (via memoryBacking <allocation mode="immediate"/>).

However, when guest's NVDIMM is backed by real life NVDIMM this
approach is not the best. In this case users should put <pmem/>
into the <memory/> device <source/>, like this:

  <memory model='nvdimm' access='shared'>
    <source>
      <path>/dev/pmem0</path>
      <pmem/>
    </source>
  </memory>

Instructing QEMU to do prealloc in this case means that each
page of the NVDIMM is "touched" (the first byte is read and
written back - see QEMU commit v2.9.0-rc1~26^2) which cripples
device wear.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1894053
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>