NVDIMM emulation will mmap the backend file, it uses host pagesize
as the alignment of mapping address before, but some backends may
require alignments different from the pagesize. So the 'alignsize'
option is introduced to allow specification of the proper alignment:

<devices>
  ...
  <memory model='nvdimm' access='shared'>
      <source>
          <path>/dev/dax0.0</path>
          <alignsize unit='MiB'>2</alignsize>
      </source>
      <target>
          <size unit='MiB'>4094</size>
          <node>0</node>
          <label>
              <size unit='MiB'>2</size>
          </label>
      </target>
  </memory>
  ...
</devices>
Signed-off-by: NLuyao Zhong <luyao.zhong@intel.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>