sheepdog: implement .bdrv_get_allocated_file_size

With this patch, qemu-img info sheepdog:image will show disk size for sheepdog
images.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Reviewed-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
sheepdog: refactor do_sd_create()

We can actually use BDRVSheepdogState *s to pass most of the parameters.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
sheepdog: support user-defined redundancy option

Sheepdog support two kinds of redundancy, full replication and erasure coding.

# create a fully replicated vdi with x copies
 -o redundancy=x (1 <= x <= SD_MAX_COPIES)

# create a erasure coded vdi with x data strips and y parity strips
 -o redundancy=x:y (x must be one of {2,4,8,16} and 1 <= y < SD_EC_MAX_STRIP)

E.g, to convert a vdi into sheepdog vdi 'test' with 8:3 erasure coding scheme

$ qemu-img convert -o redundancy=8:3 linux-0.2.img sheepdog:test

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
sheepdog: check if '-o redundancy' is passed from user

This fix a segfault (that is caused by b3af018f3) of following command:

$ qemu-img convert some_img sheepdog:some_img

Cc: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
sheepdog: fix dynamic grow for running qcow2 format

When running qcow2 over sheepdog, we might meet following problem

  qemu-system-x86_64: shrinking is not supported

And cause IO errors to Guest. This is because we abuse bs->total_sectors, which
is manipulated by generic block layer and race with sheepdog code.

We should directly check if offset > vdi_size to dynamically enlarge the volume
instead of 'offset > bs->total_sectors', which will cause problem when following
case happens:

   vdi_size > offset > bs->total_sectors

   # then trigger sd_truncate() to shrink the volume wrongly.

Cc: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: Hadrien KOHL <hadrien.kohl@gmail.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>