vhdx: Some more BlockBackend use in vhdx_create()

This does some easy conversions from bdrv_* to blk_* functions in
vhdx_create(). We should avoid bypassing the BlockBackend layer whenever
possible.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
