x86: add new board to accomodate Jailhouse port

The port will enable Zephyr to run as a guest OS on x86-64 systems. It
comes with a test on QEMU to validate that, thus this new board
introduction. It's "make run" target will issue QEMU with the same
configuration Jailhouse upstream uses for their confis/qemu-x86.c root
cell configuration:

  Test configuration for QEMU Q35 VM, 1 GB RAM, 4 cores,
  6 MB hypervisor, 60 MB inmates (-4K shared mem device)

This will work provided qemu-system-x86_64 is installed in the system
and a given (qcow2) image with the Jailhouse root cell in it is
provided (any of those will ever ship with Zephyr, it's out of its
scope).

Signed-off-by: Gustavo Lima Chaves <gustavo.lima.chaves@intel.com>