Support for TPM command line options

This patch adds support for TPM command line options.
The command line options supported here are

./qemu-... -tpmdev passthrough,path=<path to TPM device>,id=<id>
           -device tpm-tis,tpmdev=<id>,id=<other id>

and

./qemu-... -tpmdev help

where the latter works similar to -soundhw help and shows a list of
available TPM backends (for example 'passthrough').

Using the type parameter, the backend is chosen, i.e., 'passthrough' for the
passthrough driver. The interpretation of the other parameters along
with determining whether enough parameters were provided is pushed into
the backend driver, which needs to implement the interface function
'create' and return a TPMDriverOpts structure if the VM can be started or
'NULL' if not enough or bad parameters were provided.

Monitor support for 'info tpm' has been added. It for example prints the
following:

(qemu) info tpm
TPM devices:
 tpm0: model=tpm-tis
  \ tpm0: type=passthrough,path=/dev/tpm0,cancel-path=/sys/devices/pnp0/00:09/cancel

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: 1361987275-26289-2-git-send-email-stefanb@linux.vnet.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>