KVM: selftests: Rename timespec_diff_now to timespec_elapsed

In response to some earlier comments from Peter Xu, rename
timespec_diff_now to the much more sensible timespec_elapsed.

No functional change intended.

Reviewed-by: Jacob Xu <jacobhxu@google.com>
Reviewed-by: Makarand Sonare <makarandsonare@google.com>

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210112214253.463999-2-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>