sheepdog: add full data preallocation support

sheepdog: add full data preallocation support

This introduces qemu-img create option for sheepdog which allows the
data to be fully preallocated (note that sheepdog always preallocates
metadata).

The option is disabled by default and you need to enable it like the
following:

qemu-img create sheepdog:test -o preallocation=full 1G

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>