tools/kvm_stat: fix handling of invalid paths in debugfs provider

tools/kvm_stat: fix handling of invalid paths in debugfs provider

When filtering by guest, kvm_stat displays garbage when the guest is
destroyed - see sample output below.
We add code to remove the invalid paths from the providers, so at least
no more garbage is displayed.
Here's a sample output to illustrate:

  kvm statistics - pid 13986 (foo)

   Event                                         Total %Total CurAvg/s
   diagnose_258                                     -2    0.0        0
   deliver_program_interruption                     -3    0.0        0
   diagnose_308                                     -4    0.0        0
   halt_poll_invalid                               -91    0.0       -6
   deliver_service_signal                         -244    0.0      -16
   halt_successful_poll                           -250    0.1      -17
   exit_pei                                       -285    0.1      -19
   exit_external_request                          -312    0.1      -21
   diagnose_9c                                    -328    0.1      -22
   userspace_handled                              -713    0.1      -47
   halt_attempted_poll                            -939    0.2      -62
   deliver_emergency_signal                      -3126    0.6     -208
   halt_wakeup                                   -7199    1.5     -481
   exit_wait_state                               -7379    1.5     -493
   diagnose_500                                 -56499   11.5    -3757
   exit_null                                    -85491   17.4    -5685
   diagnose_44                                 -133300   27.1    -8874
   exit_instruction                            -195898   39.8   -13037
   Total                                       -492063

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>