exec.c: Use subpages for large unaligned mappings

exec.c: Use subpages for large unaligned mappings

Registering a multi-page memory region that is non-page-aligned results
in a subpage from the start to the page boundary, some number of full
pages, and possibly another subpage from the last page boundary to the
end. The full pages will have a value for offset_within_region that is
not a multiple of TARGET_PAGE_SIZE. Accesses through softmmu are unable
to handle this and will segfault.

Handling full pages through subpages is not optimal, but only
non-page-aligned mappings take the penalty.

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>