Fix subtle integer overflow bug in memory API

It is quite common to have a MemoryRegion with size of INT64_MAX.
When processing alias regions in render_memory_region() it's quite
easy to find a case where it will construct a temporary AddrRange with
a non-zero start, and size still of INT64_MAX.  When means attempting
to compute the end of such a range as start + size will result in
signed integer overflow.

This integer overflow means that addrrange_intersects() can
incorrectly report regions as not intersecting when they do.  For
example consider the case of address ranges {0x10000000000,
0x7fffffffffffffff} and {0x10010000000, 0x10000000} where the second
is in fact included completely in the first.

This patch rearranges addrrange_intersects() to avoid the integer
overflow, correcting this behaviour.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Avi Kivity <avi@redhat.com>