doc: Update info about Xen in user guide and develop manual

Xen in the v8r64 stack has been upgraded to 4.17, the main relevant
changes are:
* Changed the codebase and branch
* Put the implementation of Armv8-R AArch64 with MPU support in Xen
  as downstream patches in v8r64

Update the User Guide and Developer Manual sections with the
information related to the above changes.

Issue-Id: SCM-5624
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Signed-off-by: Qi Feng <qi.feng@arm.com>
Change-Id: I6b5f862a9ef9add427c41d1edfd898fbd4d86009