Parse the domain XML with TPM passthrough support.
The TPM passthrough XML may look like this:

    <tpm model='tpm-tis'>
      <backend type='passthrough'>
        <device path='/dev/tpm0'/>
      </backend>
    </tpm>
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: NCorey Bryant <coreyb@linux.vnet.ibm.com>
Tested-by: NCorey Bryant <coreyb@linux.vnet.ibm.com>