qemu: introduce the "virtualization" feature

qemu: introduce the "virtualization" feature

The "virt" board in QEMU has a "virtualization" option
that is documented like this:

virtualization
  Set ``on``/``off`` to enable/disable emulating a guest CPU which implements the
  Arm Virtualization Extensions. The default is ``off``.

(from system/arm/virt.rst)

According to the documentation, the "virtualiaztion" option
is related to the "gic-version" option. Specifically, gic version=4
requires virtualization to be enabled. And gic version=max will use
version=4 when virtualization is enabled, and 3 when not.
Libvirt does not currently model neither gic version "4" nor "max"
though.

It is also documented for the "vexpress-a(9|15)" boards, where it is
also disabled by default:

- QEMU defaults to providing a CPU which does not provide either
  TrustZone or the Virtualization Extensions: if you want these you
  must enable them with ``-machine secure=on`` and ``-machine
  virtualization=on``

(system/arm/vexpress.rst).

On the command line it looks like:

 qemu-system-aarch64 -machine type=virt,virtualization=on ..

Model it using the "virtualization" element in the "features" section:

  <features>
    <virtualization/>
  </features>

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>