x86/traps: Adjust paged-guest handling in the PV pagefault path

x86/traps: Adjust paged-guest handling in the PV pagefault path

PV guests necessarily can't be external, as Xen must steal address space from
them.  Pagefaults for HVM guests are handled by {vmx,svm}_vmexit_handler() and
don't enter the PV fixup_page_fault() path.  Therefore, the first call to
paging_fault() is dead, and dropped.

Logdirty mode is now the only paging mode we should ever find a PV guest with,
so add a new predicate and assertion to this fact.

Drop the final reference to paging_mode_external().  It is more accurately now
only for logdirty guests.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>