QEMU 4.1.0 introduced a new device type called TPM Proxy, currently
implemented by PPC64 guests via a new virtual device called
'spapr-tpm-proxy' (see QEMU 0fb6bd073230 for more info).

The TPM Proxy device interacts with a TPM Resource Manager, a host
device capable of multiplexing the host TPM with multiple processes.
This allows multiple guests to access some TPM features at the
same time. Note that this mode of operation does not provide
full TPM features to be available for the guest - for that case
the guest still needs to assign a vTPM device (tpm-spapr for
PPC64 guests). Although redundant, there is currently no technical
limitation for a guest to assign both a vTPM and a TPM Proxy at the
same time.

This patch adds documentation and schema for a new TPM model
type called 'spapr-tpm-proxy' that creates this new TPM Proxy
device. This model is valid only for the 'passthrough' backend.
An example of a TPM Proxy device connected to a TPM Resource Manager
'/dev/tpmrm0' will look like this:

<tpm model='spapr-tpm-proxy'>
  <backend type='passthrough'>
    <device path='/dev/tpmrm0'/>
  </backend>
</tpm>
Tested-by: NSatheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Reviewed-by: NStefan Berger <stefanb@linux.ibm.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>