Cleanup: bdrv_open() no need to shift total_size just to shift back.

In bdrv_open() there is no need to shift total_size >> 9 just to
multiply it by 512 again just a few lines later, since this is the
only place the variable is used.

Mask with BDRV_SECTOR_MASK to protect against case where we are
passed a corrupted image.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
