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>