kvm tools: Add ivshmem device

The patch adds an ivshmem device which can be used to share memory between
guests on the same host.

This implementation is lacking inter-guest communication which should be
implemented later once information regarding the client-server protocol is
gathered, though infrastructure used to add and remove clients already exists
in the patch (but isn't used anywhere).

Patch is based on David Evansky's shmem device.

Original-patch-by: David Evensky <evensky@sandia.gov>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>