tools::sgutils2: don't transmute to a Box

tools::sgutils2: don't transmute to a Box

Otherwise we run the drop handler for the scsi pt object AND
the box itself, which shouldn't even work as it should be
doing a double-free (unless the library does some kind of
reference counting in which case this should simply crash
later on?)

anyway, let's make a wrapper simply called `SgPt` containing
the pointer from `construct_scsi_pt_obj()`

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>