scripts/kvm/kvm_stat: Fix tracefs access checking

On kernels build without CONFIG_TRACING kvm_stat will bail out even
when traces are not used. This is not very helpful, especially if the
user can't install a new kernel. Instead, we should warn the user and
fall back to debugfs statistics.

These changes check if trace statistics were selected without kernel
support, warn with a small timeout, set the debugfs statistics option
to True and the tracefs one to False.

Fixes: 7aa4ee5 ('scripts/kvm/kvm_stat: Improve debugfs access checking')
Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1454485291-43849-2-git-send-email-frankja@linux.vnet.ibm.com>
[Exit if -t is passed explicitly. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
