http://lists.gnu.org/archive/html/qemu-devel/2011-05/threads.html#02162

Currently, qemu silently clips any JSON integer in the range
0x8000000000000000 - 0xffffffffffffffff (all numbers in this range
will be clipped to 0x7fffffffffffffff == LLONG_MAX).

To avoid this, pass these as signed 64 bit integers in the QMP
request.