scripts/kvm/kvm_stat: Improve debugfs access checking

Access checking with F_OK was replaced with the better readable
os.path.exists().

On Linux exists() returns False when the user doesn't have sufficient
permissions for statting the directory. Therefore the error message
now states that sufficient rights are needed when the check fails.

Also added check for /sys/kernel/debug/tracing/.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-9-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
