tools: kvm_stat: Introduce pid monitoring

Having stats for single VMs can help to determine the problem of a VM
without the need of running other tools like perf.

The tracepoints already allowed pid level monitoring, but kvm_stat
didn't have support for it till now. Support for the newly implemented
debugfs vm monitoring was also implemented.
Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/kvm/kvm_stat/kvm_stat.txt

@@ -23,8 +23,6 @@ The set of KVM kernel module trace events may be specific to the kernel version
 or architecture.  It is best to check the KVM kernel module source code for the
 meaning of events.
 
-Note that trace events are counted globally across all running guests.
-
 OPTIONS
 -------
 -1::
@@ -44,6 +42,10 @@ OPTIONS
 --debugfs::
 	retrieve statistics from debugfs
 
+-p<pid>::
+--pid=<pid>::
+	limit statistics to one virtual machine (pid)
+
 -f<fields>::
 --fields=<fields>::
 	fields to display (regex)