Allow tools to map arbitrarily large machphys_mfn_list on 32bit dom0

Allow tools to map arbitrarily large machphys_mfn_list on 32bit dom0

This permits suspend/resume to work with 32bit dom0/tools when system
memory extends beyond 160GB (and up to 1TB).

AFAICT the limit to MACH2PHYS_COMPAT_NR_ENTRIES is redundant since
that refers to a limit in 32bit guest compat mappings under 64bit
hypervisors, not userspace where there may be gigabytes of useful
virtual space available for this.

Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>