[xen][hvm][tracing] Refine hvm tracing

[xen][hvm][tracing] Refine hvm tracing

This patch does two things:
 * Allows hvm traces to take advantage of the variable-size traces
 * Adds some hvm functionality

This includes tracing actions like clts and lmsw, values of {p,mm}io reads
and writes, and making different trace records for hvm domains running in
64-bit mode if the trace record includes a virtual address.