sheepdog: fix NULL dereference in sd_create

Following command

qemu-img create -f qcow2 sheepdog:test 20g

will cause core dump because aio_context is NULL in sd_create. We should
initialize it by qemu_get_aio_context() to avoid NULL dereference.

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: Stefan Hajnoczi <stefanha@redhat.com>
