Don't return addresses that aren't properly aligned for the guest,
e.g. when the guest has a larger page size than the host. Don't
return addresses that are outside the virtual address space for the
target, by paying proper attention to the h2g/g2h macros.
At the same time, place the default mapping base for 64-bit guests
(on 64-bit hosts) outside the low 4G. Consistently interpret
mmap_next_start in the guest address space.
Signed-off-by: Richard Henderson <rth@twiddle.net>
14f24e1465