Add support for cancelling of a TPM command

This patch adds support for cancelling an executing TPM command.
In Linux for example a user can cancel a command through the TPM's
sysfs 'cancel' entry using

echo "1" > /sysfs/class/misc/tpm0/device/cancel

This patch propagates the cancellation of a command inside a VM
to the host TPM's sysfs entry.
It also uses the possibility to cancel the command before QEMU VM
shutdown or reboot, which helps in preventing QEMU from hanging while
waiting for the completion of the command.
To relieve higher layers or users from having to determine the TPM's
cancel sysfs entry, the driver searches for the entry in well known
locations.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Reviewed-by: Joel Schopp <jschopp@linux.vnet.ibm.com>
Message-id: 1361987275-26289-7-git-send-email-stefanb@linux.vnet.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>