samples: drivers: virtualization: add ivshmem doorbell sample

Provide a sample for ivshmem-doorbell running under virtualization
(QEMU). The sample relies on existing tools (ivshmem-server,
ivshmem-client) that must be run at the host (unix).

The sample can be run on userspace or kernelspace, and on arm64 (qemu,
kvm) and x86_64 (qemu) platforms.

This execution of this code is quite complex, thus the README should be
made available in the official documentation. This is why the code is a
sample and not a test.

Signed-off-by: Rodrigo Cataldo <rodrigo.cataldo@huawei.com>
Co-authored-by: Henri Xavier <datacomos@huawei.com>