linux-user: take RESERVED_VA into account for g2h_valid()

When running with -R (RESERVED_VA > 0) all guest virtual addresses
are within the [0..RESERVED_VA] range. Reflect this with g2h_valid()
too so we can safely check for boundaries of our guest address space.

This is required to have the /proc/self/maps code not show maps that
aren't accessible from the guest process's point of view.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
