linux-user: Allow -R values up to 0xffff0000 for 32-bit ARM guests

The 32-bit ARM validate_guest_space() check tests whether the
specified -R value leaves enough space for us to put the
commpage in at 0xffff0f00. However it was incorrectly doing
a <= check for the check against (guest_base + guest_size),
which meant that it wasn't permitting the guest space to
butt right up against the commpage.

Fix the comparison, so that -R values all the way up to 0xffff0000
work correctly.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
