KVM: selftests: Rework timespec functions and usage

The steal_time test's timespec stop condition was wrong and should have
used the timespec functions instead to avoid being wrong, but
timespec_diff had a strange interface. Rework all the timespec API and
its use.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>