preload: Don't read udev cache data from system

Never read the udev cache data from the system, as this might make
properties of system devices leak through our mocked udev, such as
headsets showing up as internal devices.

Fixes #213

src/libumockdev-preload.c

@@ -185,6 +185,8 @@ trap_path(const char *path)
 
     if (strncmp(abspath, "/dev/", 5) == 0 || strcmp(abspath, "/dev") == 0 || strncmp(abspath, "/proc/", 6) == 0)
 	check_exist = 1;
+    else if (strncmp(abspath, "/run/udev/data", 14) == 0)
+	check_exist = 0;
     else if (strncmp(abspath, "/sys/", 5) != 0 && strcmp(abspath, "/sys") != 0)
 	return path;