[doc] module: Modify libvirt commandline block to use JSON

Latest versions of QEMU are running out of PCIe mapping
through libvirt as the commandline device is mapping
ivshmem before other root devices.
Use JSON style configuration in the commandline block.
JSON arguments are loaded after all regular ones
and will guarantee to get loaded after all other libvirt mapped
PCIe devices.

doc/module.rst

@@ -172,16 +172,23 @@ Create the following XML block in your domain:
 
    <qemu:commandline>
      <qemu:arg value='-device'/>
-     <qemu:arg value='ivshmem-plain,id=shmem0,memdev=looking-glass'/>
+     <qemu:arg value='{"driver":"ivshmem-plain","id":"shmem0","memdev":"looking-glass"}'/>
      <qemu:arg value='-object'/>
-     <qemu:arg value='memory-backend-file,id=looking-glass,mem-path=/dev/kvmfr0,size=32M,share=yes'/>
+     <qemu:arg value='{"qom-type":"memory-backend-file","id":"looking-glass","mem-path":"/dev/kvmfr0","size":33554432,"share":true}'/>
    </qemu:commandline>
 
 .. note::
 
-   Remember to add ``xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'``
+   - Remember to add ``xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'``
      to the ``<domain>`` tag.
 
+   - The ``"size"`` tag represents the size of the shared memory device in bytes.
+     Once you determine the proper size of the device as per 
+     :ref:`Determining Memory <client_determining_memory>`, use the figure you got
+     to calculate the size in bytes:
+     
+     ``size_in_MB x 1024 x 1024 = size_in_bytes``
+
 Running libvirt this way violates AppArmor and cgroups policies, which will
 block the VM from running. These policies must be amended to allow the VM
 to start: