xenpv: do not use low 1MB for Xen mappings on i386

On amd64 we already avoid using memory below 4GB in order to prevent
clashes with MMIO regions, but i386 was allowed to use any hole in
the physical memory map in order to map Xen pages.

Limit this to memory above the 1MB boundary on i386 in order to avoid
clashes with the MMIO holes in that area.

Sponsored by: Citrix Systems R&D
MFC after: 1 week