We already use the '-nodefaults' command line arg with QEMU to stop
it adding any default devices to guests. Unfortunately, QEMU will
load global config files from /etc/qemu that may also add default
devices. These aren't blocked by '-nodefaults', so we need to also
add the '-nodefconfig' arg to prevent that.

Unfortunately these global config files are also used to define
custom CPU models. So in blocking global hardware device addition
we also block definitions of new CPU models. Libvirt doesn't know
about these custom CPU models though, so it would never make use
of them anyway. Thus blocking them via -nodefconfig isn't a show
stopping problem. We would need to expand libvirt's own CPU model
XML database to support these instead.

* src/qemu/qemu_conf.c: Add '-nodefconfig' if available
* tests/qemuxml2argvdata/: Add '-nodefconfig' to all data files which
  have '-nodefaults' present

Replace

   -balloon virtio

With

   -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3

This allows it to get correct assigned PCI address as declared in
previous patch

 * src/qemu/qemu_conf.c: Convert Virtio ballon to -device and
   give it an explicit PCI address
 * tests/qemuxml2argvdata/qemuxml2argv-*args: Add in virtio balloon
   where appropriate

The current character device syntax uses either

  -serial tty,path=/dev/ttyS2

Or

  -chardev tty,id=serial0,path=/dev/ttyS2 -serial chardev:serial0

With the new -device support, we now prefer

  -chardev file,id=serial0,path=/tmp/serial.log -device isa-serial,chardev=serial0

This patch changes the existing -chardev syntax to use this new
scheme, and fallbacks to the old plain -serial syntax for old
QEMU.

The monitor device changes to

  -chardev socket,id=monitor,path=/tmp/test-monitor,server,nowait -mon chardev=monitor

In addition, this patch adds --nodefaults, which kills off the
default serial, parallel, vga and nic devices. THis avoids the
need for us to explicitly turn each off

Change -monitor, -serial and -parallel output to use -chardev if it is
available.
* src/qemu/qemu_conf.c: Update qemudBuildCommandLine to use -chardev where
  available.
* tests/qemuxml2argvtest.c tests/qemuxml2argvdata/: Add -chardev equivalents
  for all current serial and parallel tests.