netfront pv driver building

["Jan Beulich" <jbeulich@novell.com>]

netfront requires the symbol __supported_pte_mask when PAE is enabled in the
kernel being built for. That symbol, however, isn't being exported, and it doesn't
seem likely that mainline would want to see this get exported (after all, the
general assumption is that the page table handling inline functions and macros
are supposed to be used only by the memory management code).

Additionally, both uses are inside a conditional depending upon
!XENFEAT_auto_translated_physmap, which hence can't ever be true if this
code is being built as pv driver. Wouldn't it hence make sense to #ifdef these
code blocks, or to enhance xen/features.h so that checks for those features
that are always on in hvm and return a constant 1 rather than using the
xen_features array.

Thanks, Jan