x86/vmx: Add Intel Processor Trace support

x86/vmx: Add Intel Processor Trace support

Add CPUID/MSR enumeration details for Processor Trace.  For now, we will only
support its use inside VMX operation.  Fill in the vmtrace_available boolean
to activate the newly introduced common infrastructure for allocating trace
buffers.

For now, Processor Trace is going to be operated in Single Output mode behind
the guests back.  Add the MSRs to struct vcpu_msrs, and set up the buffer
limit in vmx_init_ipt() as it is fixed for the lifetime of the domain.

Context switch the most of the MSRs in and out of vCPU context, but the main
control register needs to reside in the MSR load/save lists.  Explicitly pull
the msrs pointer out into a local variable, because the optimiser cannot keep
it live across the memory clobbers in the MSR accesses.

Signed-off-by: Michał Leszczyński <michal.leszczynski@cert.pl>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>