Implement vnet_null device

Such device don't need new threads and files but are valid ethernet
devices for the kernel. Convenient for testing.

vdisk/raw.c

@@ -54,7 +54,7 @@ vdisk_raw_write(void *userdata, void *buffer, off_t startsector,
 }
 
 struct vdisk*
-vdisk_init_raw(const char *fname, struct umka_io *io) {
+vdisk_init_raw(const char *fname, const struct umka_io *io) {
     int fd = open(fname, O_RDONLY | O_BINARY);
     if (!fd) {
         printf("[vdisk.raw]: can't open file '%s': %s\n", fname, strerror(errno));