parallels: extend parallels format header with actual data values

parallels: extend parallels format header with actual data values

Parallels image format has several additional fields inside:
- nb_sectors is actually 64 bit wide. Upper 32bits are not used for
  images with signature "WithoutFreeSpace" and must be explicitly
  zeroed according to Parallels. They will be used for images with
  signature "WithouFreSpacExt"
- inuse is magic which means that the image is currently opened for
  read/write or was not closed correctly, the magic is 0x746f6e59
- data_off is the location of the first data block. It can be zero
  and in this case data starts just beyond the header aligned to
  512 bytes. Though this field does not matter for read-only driver

This patch adds these values to struct parallels_header and adds
proper handling of nb_sectors for currently supported WithoutFreeSpace
images.

WithouFreSpacExt will be covered in next patches.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Jeff Cody <jcody@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>