kvm tools: Fix type mismatches on GCC 4.4 on 32-bit systems

The tools/kvm build still fails on 32-bit:

 cc1: warnings being treated as errors
 qcow.c: In function ‘qcow1_write_sector’:
 qcow.c:307: error: comparison between signed and unsigned integer expressions
 make: *** [qcow.o] Error 1
 make: *** Waiting for unfinished jobs....

using:

 gcc version 4.4.4 20100630 (Red Hat 4.4.4-10) (GCC)

The patch below addresses them.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Pekka Enberg <penberg@kernel.org>