samples: drivers: ipm: added IPM over IVSHMEM sample

To demonstrate how to configure Zephyr to use the IPM
driver over the IVSHMEM subsystem. Since this driver
is intended to generate inter QEMU VM notifications
it was better to create a sample app using the shell
for quick demonstration for the users.

Signed-off-by: Felipe Neves <felipe.neves@linaro.org>