sheepdev: linux kernel module of block device driver for sheepdog

This module provides function for users to take sheepdog VDIs as block
devices in linux, users can register a VDI to its kernel space, it just
like that a new hard disk is added to the computer, users can create
partitions for the disk, format the disk or mount the disk, it provides
users a efficient way to use sheepdog as distributed storage system.

The usage is easy, after install the module sheepdev.ko, it creates a proc
entry '/proc/entry', you can write into the proc entry file to control
the driver.

Add a new block device from an existing sheepdog VDI:

# echo "add 127.0.0.1:7070 linux" > /proc/sheep

It would create a block device /dev/sheepa, you can format/mount this device:

# mkfs.ext4 /dev/sheepa
# mount -t ext4 /sheep/sheepa test

Remove a block device from the kernel:

# echo "del linux" > /proc/sheep
sheepdev: make it safe to open a snapshot VDI 

In case we open a snapshot VDI to make it as a block device, we immediately
create a new writable VDI for this snapshot just as what QEMU does, then we
can safely write to this VDI
sheepdev: add kbuild and Makefile.in