xend: earlier remove the backend of tapdisk device in

xend: earlier remove the backend of tapdisk device in
xenstore to release the resource allocated in backend driver
lies in dom0'kernel

Blktapctl thread will use qemu-dm connection instead of tapdisk-ioemu
in the case of FV VM. We found the resource like memory allocated for
this Guest can't be free for backend driver couldn't be closed in qemu-dm.

This patch would  remove the backend of tapdisk device earlier in
xenstore to triger qemu-dm to notify the backend driver to release the
resource allocated.
I have tested this patch at the case of
1, save && restore
2, destory && shutdown
3, snapshot

Signed-off-by: James ( Song Wei ) <jsong@novell.com>