qemu_img: is_not_zero() optimization

qemu_img: is_not_zero() optimization

I run qemu-img under profiler and realized, that most of CPU time is
consumed by is_not_zero() function. I had made a couple of optimizations
on it and got the following output for `time qemu-img convert -O qcow2
volume.qcow2 snapshot.qcow2`:

Original qemu-img:
real 0m56.159s
user 0m34.670s
sys  0m12.079s

Patched qemu-img:
real 0m34.805s
user 0m18.445s
sys  0m12.552s

Signed-off-by: Dmitry Konishchev <konishchev@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>