x86: nVMX: Print more (accurate) info if RDTSC diff test fails

Snapshot the delta of the last run and display it in the report if the
test fails.  Abort the run loop as soon as the threshold is reached so
that the displayed delta is guaranteed to a failed delta.  Displaying
the delta helps triage failures, e.g. is my system completely broken or
did I get unlucky, and aborting the loop early saves 99900 runs when
the system is indeed broken.

Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Aaron Lewis <aaronlewis@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200124234608.10754-1-sean.j.christopherson@intel.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Reviewed-by: Aaron Lewis <aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>