tools/kvm_stat: fix display of error when multiple processes are found

tools/kvm_stat: fix display of error when multiple processes are found

Instead of printing an error message, kvm_stat script fails when we
restrict statistics to a guest by its name and there are multiple guests
with such name:

  # kvm_stat -g my_vm
  Traceback (most recent call last):
    File "/usr/bin/kvm_stat", line 1819, in <module>
      main()
    File "/usr/bin/kvm_stat", line 1779, in main
      options = get_options()
    File "/usr/bin/kvm_stat", line 1718, in get_options
      options = argparser.parse_args()
    File "/usr/lib64/python3.10/argparse.py", line 1825, in parse_args
      args, argv = self.parse_known_args(args, namespace)
    File "/usr/lib64/python3.10/argparse.py", line 1858, in parse_known_args
      namespace, args = self._parse_known_args(args, namespace)
    File "/usr/lib64/python3.10/argparse.py", line 2067, in _parse_known_args
      start_index = consume_optional(start_index)
    File "/usr/lib64/python3.10/argparse.py", line 2007, in consume_optional
      take_action(action, args, option_string)
    File "/usr/lib64/python3.10/argparse.py", line 1935, in take_action
      action(self, namespace, argument_values, option_string)
    File "/usr/bin/kvm_stat", line 1649, in __call__
      ' to specify the desired pid'.format(" ".join(pids)))
  TypeError: sequence item 0: expected str instance, int found

To avoid this, it's needed to convert pids int values to strings before
pass them to join().

Signed-off-by: Dmitry Klochkov <kdmitry556@gmail.com>
Message-Id: <20220614121141.160689-1-kdmitry556@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>