drivers/virtualization: Adding ivshmem driver

This is placed into drivers/virtualization as it does not belong to any
existing subsystem.

This is only the ivshmem-plain variant.

This device is provided by qemu or ACRN, and can be used to share memory
either between the host and the VM or between VMs. Here if zephyr is
used as a VM, it will be able to take advantage of such feature.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>