test-coroutine: avoid overflow on 32-bit systems

unsigned long is not large enough to represent 1000000000 * duration there.
Just use floating point.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1417518350-6167-4-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
