exec.c: Fix off-by-one error in register_subpage

subpage_register() expects "end" to be the last byte in the mapping.
Registering a non-page-aligned memory region that extends up to or
beyond a page boundary causes subpage_register() to silently fail
through the (end >= PAGE_SIZE) check.

This bug does not cause noticeable problems for mappings that do not
extend to a page boundary, though they do register an extra byte.

Signed-off-by: Tyler Hall <tylerwhall@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
