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>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
