sheepdog: selectable object size support

sheepdog: selectable object size support

Previously, qemu block driver of sheepdog used hard-coded VDI object size.
This patch enables users to handle VDI object size.

When you start qemu, you don't need to specify additional command option.

But when you create the VDI which doesn't have default object size
with qemu-img command, you specify object_size option.

If you want to create a VDI of 8MB object size,
you need to specify following command option.

 # qemu-img create -o object_size=8M sheepdog:test1 100M

In addition, when you don't specify qemu-img command option,
a default value of sheepdog cluster is used for creating VDI.

 # qemu-img create sheepdog:test2 100M

Signed-off-by: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
Acked-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>