kvm tools: remove unneeded checks in qcow code

We already know q!=NULL at that point, no need to retest.

Reviewed-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>