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.

src/core/import-creds.c

@@ -713,5 +713,18 @@ int import_credentials(void) {
                         r = q;
         }
 
+        if (r >= 0) {
+                _cleanup_free_ char *address = NULL;
+
+                r = read_credential("vmm.notify_socket", (void **)&address, /* ret_size= */ NULL);
+                if (r < 0 && !IN_SET(r, -ENOENT, -ENXIO))
+                        log_warning_errno(r, "Failed to read 'vmm.notify_socket' credential, ignoring: %m");
+                else if (r >= 0 && !isempty(address)) {
+                        r = setenv("NOTIFY_SOCKET", address, /* replace= */ 1);
+                        if (r < 0)
+                                log_warning_errno(errno, "Failed to set $NOTIFY_SOCKET environment variable, ignoring: %m");
+                }
+        }
+
         return r;
 }