kvm tools: Switch to using UNIX sockets instead of signals

This patch changes the IPC method to use UNIX sockets instead of
signals.

This allows for more flexibility in moving data between processes, for example
it would allow avoid printing output in the terminal of the guest.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>