[xentrace][HVM] introduce HVM tracing to unify SVM and VMX tracing

[xentrace][HVM] introduce HVM tracing to unify SVM and VMX tracing

* new tracing class TRC_HVM replacing TRC_VMX

* 20 new trace events to differentiate the information carried by them
** added corresponding trace points in SVM and VMX code
** updated formats file: descriptive formatting of trace event data
** completely replacing previous simple tracing in VMX

* possibility to toggle single events on and off at compile time in
  include/asm-x86/hvm/trace.h

Signed-off-by: Thomas Friebel <thomas.friebel@amd.com>