Since libvirt for dubious historical reasons stores memory size as
kibibytes, it's possible that the alignments done in the qemu code
overflow the the maximum representable size in bytes. The XML parser
code handles them in bytes in some stages. Prevent this by doing
overflow checks when alinging the size and add a test case.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1260576