tools/kvm_stat: fix command line option '-g'

Specifying a guest via '-g foo' always results in an error:
  $ kvm_stat -g foo
  Usage: kvm_stat [options]

  kvm_stat: error: Error while searching for guest "foo", use "-p" to
  specify a pid instead

Reason is that Tui.get_pid_from_gname() is not static, as it is supposed
to be.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>