EPT: utilize GLA->GPA translation known for certain faults

EPT: utilize GLA->GPA translation known for certain faults

Rather than doing the translation ourselves in __hvmemul_{read,write}()
leverage that we know the association for faults other than such having
occurred when translating addresses of page tables.

There is one intentional but not necessarily obvious (and possibly
subtle) adjustment to behavior: __hvmemul_read() no longer blindly
bails on instruction fetches matching the MMIO GVA (the callers of
handle_mmio_with_translation() now control the behavior via the struct
npfec they pass, and it didn't seem right to bail here rather than just
falling through to the unaccelerated path)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>