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>