ueventd: support by-name symlinks for virtual block devices

This patch makes ueventd create by-name symlinks for virtual block
devices when Android is running as a Xen guest OS.

The symlinks will be created under /dev/block/vbd/. For example:
/dev/block/vbd/768/by-name/

Change-Id: Id45da560d13f641d7c8419511c154de9ae8a4ec9
Test: manual
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>