x86/traps: Correct pagefault handling issues introduced in c/s d5c251c

x86/traps: Correct pagefault handling issues introduced in c/s d5c251c

There are two bugs.

Firstly, the ASSERT(paging_mode_only_log_dirty(d)) can trip when servicing a
hypervisor #PF in the context of an HVM guest, e.g. a copy_to_user() failure
in the shadow pagetable code.

Secondly, the entry conditions paging_fault() were previously guarded on
!paging_mode_external(d) which limited entry to PV contexts, but for both
guest and hypervisor faults.  Switching this to paging_mode_log_dirty() opened
it up to HVM contexts as well.

Reinstate the old !paging_mode_external(d) check, as it is actually the
relevent fact, and extend the comment to explicitly state that hypervisor
faults should follow this path.

Inside, we are now guarenteed to be in the context of a PV guest, so can
safely use the assertion about log dirty.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>