veth: Factor out veth conf freeing

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
net: Add API call for removing nets from configuration

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fs: Add API call for removing external bind mounts

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>