hmp: add basic "info stats" implementation

Add an HMP command to retrieve statistics collected at run-time.
The command will retrieve and print either all VM-level statistics,
or all vCPU-level statistics for the currently selected CPU.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>