creds: import 'vmm.notify_socket' and use it to set

creds: import 'vmm.notify_socket' and use it to set

This is intended to be used with VSOCK, to notify the hypervisor/VMM, eg on the host:

qemu <...> -smbios type=11,value=io.systemd.credential:vmm.notify_socket=vsock:2:1234 -device vhost-vsock-pci,id=vhost-vsock-pci0,guest-cid=42

(vsock:2:1234 -> send to host on vsock port 1234, default is to send to 0 which is
the hypervisor itself)

Also on the host:

$ socat - VSOCK-LISTEN:1234,socktype=5
READY=1
STATUS=Ready.