qemu: add dtb option support

qemu: add dtb option support

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: Eric Blake <eblake@redhat.com>