x86/pv: Code improvements to do_update_descriptor()

x86/pv: Code improvements to do_update_descriptor()

 * Add "uint64_t raw" to seg_desc_t to remove the opencoded uint64_t casting
   in this function.  Change the parameter to be of type seg_desc_t.
 * Rename the 'pa' parameter to 'gaddr', because it lives in GFN space rather
   than physical address space.
 * Use gfn_t and mfn_t rather than unsigned longs.
 * Check the alignment and proposed new descriptor before taking a page
   reference.
 * Use the more flexible ACCESS_ONCE() accessor in preference to
   write_atomic()

No expected change in behaviour.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>