ppc64 guests don't require adding a NUMA node for hotplug memory to
work. Lift the requirement and add test cases.

ppce500v2 is not machine supported by official release of QEMU.
It should be replaced by ppce500.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

The "dtb" option sets the filename for the device tree.
If without this option support, "-dtb file" will be converted into
<qemu:commandline> in domain XML file.
For example, '-dtb /media/ram/test.dtb' will be converted into
  <qemu:commandline>
    <qemu:arg value='-dtb'/>
    <qemu:arg value='/media/ram/test.dtb'/>
  </qemu:commandline>

This is not very friendly.
This patchset add special <dtb> tag like <kernel> and <initrd>
which is easier for user to write domain XML file.
  <os>
    <type arch='ppc' machine='ppce500v2'>hvm</type>
    <kernel>/media/ram/uImage</kernel>
    <initrd>/media/ram/ramdisk</initrd>
    <dtb>/media/ram/test.dtb</dtb>
    <cmdline>root=/dev/ram rw console=ttyS0,115200</cmdline>
  </os>
Signed-off-by: NEric Blake <eblake@redhat.com>