Parse TPM passthrough XML in the domain XML

Parse TPM passthrough XML in the domain XML

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: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Tested-by: Corey Bryant <coreyb@linux.vnet.ibm.com>