For pSeries guest in QEMU, NVRAM is one kind of spapr-vio device.
Users are allowed to specify spapr-vio devices'address.
But NVRAM is not supported in libvirt. So this patch is to
add NVRAM device to allow users to specify its address.

In QEMU, NVRAM device's address is specified by
 "-global spapr-nvram.reg=xxxxx".

In libvirt, XML file is defined as the following:

  <nvram>
    <address type='spapr-vio' reg='0x3000'/>
  </nvram>
Signed-off-by: NLi Zhang <zhlcindy@linux.vnet.ibm.com>