[LIBS] vfs: Auto-unmount block device when it is unregistered.

This patch makes VFS listen to block device events to implement auto-unmount on unregister event.

The auto-unmount is very importand feature required in VFS for hot-pluggable devices such as MMC/SD card and USB mass storage devices.

Signed-off-by: Anup Patel <anup@brainfault.org>